Merge i.MX and PowerPC SDHCI drivers

Summary:
i.MX5 and PowerPC use a very similar eSDHC controller, which is also
similar to the uSDHC controller used by i.MX6. The imx_sdhci driver works
almost completely with PowerPC, with some minor tweaks.

There is one caveat with this: reset currently does not work on PowerPC, so has
been #ifdef'd out until this can be tracked down and fixed. If resets are done
the controller will timeout all data transactions. Without a reset, it appears
to work just fine.

This is part 3, following up r308186 and r308187.

Test Plan:
This has been tested on a PowerPC QorIQ P1022 board. It has not been
tested on i.MX, but no regressions are expected.

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D8407