omap: Enable GPMC clock in gpmc_init

Don't assume that gpmc_l3_clk is on, enable it before touching
configuration registers.

Note that the current code assumes that this clock is always
enabled. We are already setting smart idle and L3 autogating
for GPMC clock in gpmc_init.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>