spmi: pmic-arb: Correct the mapping table size

spmi pmic arbiter V2 supports 256 peripherals. But the size
of the spmi peripheral mapping table in the driver is defined
as 255, which leads to the buffer overflow. Fix it by correcting
the mapping table size to 256.

Change-Id: I096e35bae470a56af94edca98e3e793cf3ec1751
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>