msm: pcie: control when PCIe MSI driver can access SNPS registers

It is possible PCIe MSI driver will try to access PCIe Synopsys
MSI registers while there is no clock provided. Have PCIe core
driver control when PCIe MSI driver can access such registers.

Change-Id: I74e5fd27a7af4a84eb645d91a3eff890cf3d84d0
Signed-off-by: Tony Truong <truong@codeaurora.org>