firmware: arm_scpi: increase MAX_DVFS_OPPS to 16 entries

Since Amlogic SoCs supports more than 8 OPPs per domains, we need increase
the OPP structure size.

This patch increases the MAX_DVFS_OPPS to 16.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>