ARM: dts: DRA7: use new dra7-specific compatible string

Use the new compatible string "ti,dra7-hsmmc" that was specifically
added for dra7 and dra72. This is required since for dra7 and dra72
processors iodelay values has to be set unlike other processors.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>