Add a driver for the SDX55 APCS clock controller. It is part of the APCS
hardware block, which among other things implements also a combined mux
and half integer divider functionality. The APCS clock controller has 3
parent clocks:

1. Board XO
2. Fixed rate GPLL0
3. A7 PLL

This is required for enabling CPU frequency scaling on SDX55-based
platforms.
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20210118041156.50016-6-manivannan.sadhasivam@linaro.org
[sboyd@kernel.org: Fix unused ret in probe by hardcoding it]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>