arch: omap: add userspace export of the current GPU clock. Works mostly like scaling_cur_freq.
Source: https://github.com/franciscofranco/Tuna_JB_pre1/commit/a9c8946aef889caaed0f37c8caf475ccdece78f5

Conflicts:
	arch/arm/mach-omap2/omap2plus-cpufreq.c
omap2: fix some warnings

Signed-off-by: Willi Ye <williye97@gmail.com>