ath10k: add clock ctrl related functions in ahb

pre qca4019 chipsets has/uses internal clock generator for
the operation. But, qca4019 uses external clocks supplied from
outside of target (ie, outside of wifi core). Three different clocks
(cmd clock, ref clock, rtc clock) comes into picture in qca4019.
All those clocks needs to configured with help of global clock
controller (gcc) to make qca4019 functioning.

Add functions for clock init/deinit, clock enable/disable in ahb.
This is just a preparation, functions added in this patch will be
used in later patches.

Signed-off-by: Raja Mani <rmani@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>