sh: pfc: Variable bitfield width config register support

Add support for variable config reg hardware by adding
the macro PINMUX_CFG_REG_VAR(). The width of each bitfield
needs to be passed to the macro, and the correct space must
be consumed by each bitfield in the enum table following the
macro. Data registers still need to have fixed bitfields.

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