ARM: OMAP: hsmmc: Add support for MMC 2 setup for AM35x

AM35x MMC 2 controller has internal clock loopback setting which cannot
be utilized without this patch and thus SDIO devices connected to this
controller and depend on this setting will fail to initialize.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>