clk: qcom: Add A53 clock driver

Add a driver for the A53 Clock Controller. It is a hardware block that
implements a combined mux and half integer divider functionality. It can
choose between a fixed-rate clock or the dedicated A53 PLL. The source
and the divider can be set both at the same time.

This is required for enabling CPU frequency scaling on platforms like
MSM8916.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>