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, it can be defined if SPMI debugfs write is not desired.

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