clk: mmp: add PXA1928 clock support

Add initial clock support for Marvell PXA1928. The PXA1928 is a mobile
SOC and is similar to other MMP/PXA series of SOCs, so a lot of the
existing infrastructure is reused here.

Currently the PLLs are just fixed clocks, and not all leaf clocks are
implemented.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>