OMAP4: prm: Replace warm reset API with the offset based version

The warm reset function was still using the obsolete API.
Replace it by the new one and move the file to the proper c file.

Change the function names to stick to the file convention as
suggested by Paul Walmsley <paul@pwsan.com>:
prm_xxx -> prminst_xxx

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>