ath10k: add helper functions in ahb.c for reg rd/wr

qca4019 deals with below register memory region to control the clock,
reset, etc.

        - Memory to control wifi core
        - gcc (outside of wifi)
        - tcsr (outside of wifi)

Add new helper functions to perform read/write in above registers
spaces. Actual ioremap for above registers are done in later patch.
Struct ath10k_ahb is introduced to maintain ahb specific info and
memory this struct will be allocated in the continuation of struct
ath10k_pci (again, memory ath10k_ahb is allocated in the later patch).

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