spmi: pmic-arb: Correct the peripheral mapping table size

spmi pmic arbiter V2 supports 256 peripherals. But the size
of the 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: I88f1a01f28ad831811d3865b748beb1cba678c7d
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>