ARM: OMAP4+: PRM: determine prm_device_inst based on DT compatibility

PRM device instance offset is now provided through the prm_init_data.
This gets rid of some cpu_is_X / soc_is_X calls from PRM core code,
preparing for PRM to be its own separate driver.

Signed-off-by: Tero Kristo <t-kristo@ti.com>