This patch implements an idea from Igor Stoppa <igor.stoppa@nokia.com>
from last year.  We use functions to modify PRM/CM register bits,
rather than open-coding those operations.

The patch provides functions that do read + AND + OR + write sequences
on CM and PRM registers: {cm,prm}_rmw_reg_bits(), and
{cm,prm}_rmw_mod_reg_bits().  Several convenience functions are then
implemented on top of those functions for setting and clearing bits:
{cm,prm}_{set,clear}_mod_reg_bits().

These functions don't provide any locking; it is expected that the caller
will handle this.

Thanks to Jouni Högander <jouni.hogander@nokia.com> for catching some
embarrassing errors in earlier versions of this code.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>