OMAP4: hwmod: Replace RSTCTRL absolute address with offset macros

The RSTCTRL register was accessed using an absolute address.
The usage of hardcoded macros to calculate virtual address from physical
one should be avoided as much as possible.
The usage of an offset will allow future improvement like migration from
the current architecture code toward a module driver.

Update prm_xxx accessors, move definition to the proper header file and
update copyrights.
Change the s16 register offset parameter to u16.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>
[paul@pwsan.com: use '_prminst_' in function names that are part of the
 prminst44xx.c file]
Signed-off-by: Paul Walmsley <paul@pwsan.com>