soc: qcom: Separate kryo l2 accessors from PMU driver

The driver provides kernel level API for other drivers
to access the MSM8996 L2 cache registers.
Separating the L2 access code from the PMU driver and
making it public to allow other drivers use it.
The accesses must be separated with a single spinlock,
maintained in this driver.

Signed-off-by: Ilia Lin <ilialin@codeaurora.org>