dt-bindings: ti,omap-hsmmc: Add 66AK2G mmc controller

Update the ti,omap-hsmmc.txt to include information about
66AK2G specific mmc controller. Also cleanup the entries
under optional properties to look a bit nicer.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>