Fix NRF52 SPI pin initialization

[marcuschangarm]

SPI pins are not initialized correctly according to the current SPI mode.

This commit changes how the SPI instance is configured and ensures
that:

1. SPI pins are initialized on each object initialization.
2. SPI pins are reinitialized on each mode or object change.
3. SPI instance is only initialized when a change has occured.

Description

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1842
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6717/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1488
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6717/

[mbed-ci]

Test : SUCCESS

Build number : 1651
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6717/1651

[adbridge]

NRF5x does not exist on the 5.8 branch... Came in on this PR #6547, for 5.9