clk: mmp: pxa1908: Instantiate power driver through auxiliary bus

The power domain driver shares the APMU clock controller's registers.
Instantiate the power domain driver through the APMU clock driver using
the auxiliary bus.

Also create a separate Kconfig entry for the PXA1908 clock driver to
allow (de)selecting the driver at will and selecting
CONFIG_AUXILIARY_BUS.

Signed-off-by: Duje Mihanović <duje@dujemihanovic.xyz>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>