ARM: mx51: Add entry for gpc_dvfs_clk

For MX51 SRPG, we need to turn on the GPC clock in order to set the
SRPG registers.

Signed-off-by: Dinh Nguyen <Dinh.Nguyen@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>