mmc: sdhci_am654: Add Support for TI's AM642 SoC

Add support for the controller present on the AM642 SoC.

There are instances:
sdhci0: 8bit bus width, max 400 MBps
sdhci1: 4bit bus width, max 100 MBps

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>