rust: add initial common clock framework bindings

[m-falkowski1]

This patch adds initial abstractions including:

• Clk wrapper around struct clk *.
• Binding of clk_get() method implemented
  once as an associated method of Clk type
  and second as a method of device::RawDevice trait.
• Routines get_rate() and prepare_enable(),
  implemented as methods of Clk type.

This patch is a dependency of a Samsung Exynos trng driver provided initially in #554.

Changes v3 -> v4:

• Removed unnecessary method Clk::get(). rust: add initial common clock framework bindings #593 (comment)
• Documentation improvement rust: add initial common clock framework bindings #593 (comment)
• Added EnableClk variant of the Clk type that manages the use
  of the bindings::clk_disable_unprepare() function. rust: add initial common clock framework bindings #593 (comment)

Changes v2 -> v3:

• Binding of devm_clk_get() has been removed and replaces
  with bindings::clk_get() variant. rust: add initial common clock framework bindings #593 (comment)
• Implemented Drop trait for Clk structure that calls bindings::clk_put.
• (*self).0 -> self.0. rust: add initial common clock framework bindings #593 (comment)
• Used to_result() method instead of checking error code explicitly. rust: add initial common clock framework bindings #593 (comment)
• id field of Clk::get() method is now optional, precisely Option<&CStr>. rust: add initial common clock framework bindings #593 (comment) and rust: add initial common clock framework bindings #593 (comment)
• Added a variant of Clk::get() to device::RawDevice trait. rust: add initial common clock framework bindings #593 (comment)
• Docs fixes. rust: add initial common clock framework bindings #593 (comment)

Changes v1 (#554) -> v2:

• Added invariant description for Clk type Add Samsung Exynos true random number generator driver #554 (comment)
• Made prepare_enable() a method of Clk type Add Samsung Exynos true random number generator driver #554 (comment)
• Added #[cfg(CONFIG_COMMON_CLK)] in a module declaration at rust/kernel/lib.rs.
• devm_clk_get() uses now device::RawDevice to get access to a raw device pointer based on 6f94f6a.

Signed-off-by: Maciej Falkowski [email protected]

[m-falkowski1]

The new version has just been uploaded. Feel free to review.

[wedsonaf]

Thanks for following up on this. This is looking good.

[m-falkowski1]

The v4 version has just been uploaded. Feel free to review.