zynq7000: add platformctl for setting SD card pins

[jmaksymowicz]

This PR adds a platformctl command for selecting WP (write protect) and CD (card detect) pins for the SDIO controller.

Description

On Zynq 7000 platform the SDIO controller can handle signals from card detect and write protect pins of an SD card slot. Selection of which GPIO pins serve these functions is done through SDx_WP_CD_SEL register in slcr register bank. This PR adds a way of manipulating these registers consistent with other slcr registers.

Motivation and Context

Necessary for implementing a fully-featured SD Host Controller driver.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: armv7a9-zynq7000-zturn

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

5 734 tests  +13   5 095 ✔️ +12   28m 26s ⏱️ + 1m 49s
   311 suites +13      639 💤 +  1 
       1 files   ±  0          0 ❌ ±  0 

Results for commit 25a18c8. ± Comparison against base commit 9edeea5.

♻️ This comment has been updated with latest results.

[agkaminski]

Nitpicks, otherwise looks good to me