sh: pfc: Add config_reg_helper() function

Add a helper function for shared config reg access
calculations. This allows us to reduce the amount
of duplicated code, and at the same time prepare
for a common place for future variable bitwidth
config reg support.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>