platform: msm: spmi: Add flag to make SPMI debugfs write optional

Currently, the SPMI debugfs entries support peek/poke functionality to
access PMIC register directly. This is a possible way to access the PMIC
and change some unexpected setting which is very dangerous. Add a flag
CONFIG_MSM_SPMI_DEBUGFS_RO to make the write operation through debugfs
optional, and make SPMI debugfs write default disabled.

CRs-Fixed: 1103178
Change-Id: Iedbbf57a4a24a18f755a8b304591cc151b10bd4c
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>