ARM: OMAP4: Fix clock_get error for GPMC during boot

Looks like we still have the legacy clock alias name for
omap4 GPMC (General Purpose Memory Controller), so let's
fix it for the device tree naming. There's no need to keep
the legacy naming as omap4 is DT only nowadays.

Without this fix we get the following error while booting:

[    0.440399] omap-gpmc 50000000.gpmc: error: clk_get

Reported-by: Olof Johansson <olof@lixom.net>
Cc: stable@vger.kernel.org # v3.11
Signed-off-by: Tony Lindgren <tony@atomide.com>