spelling.txt: add more spellings to spelling.txt

Here are some of the more common spelling mistakes and typos that I've
found while fixing up spelling mistakes in the kernel over the past 6
months.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Colin Ian King <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Colin Ian King