include: sbi: Define SBI_PMU_HW_EVENT_MAX to 256

Increase maximum number of PMU hardware events that can be mapped
by OpenSBI to 256

Signed-off-by: Jun Liang Tan <junliang.tan@linux.starfivetech.com>
Signed-off-by: Wei Liang Lim <weiliang.lim@linux.starfivetech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>