[ARM] OMAP2: Fix definition of SGX clock register bits

The GFX/SGX functional and interface clocks have different masks, for
some unknown reason, so split EN_SGX_SHIFT into one each for fclk and
iclk.

Correct according to the TRM and the far more important 'does this
actually work at all?' metric.

linux-omap source commit is de1121fdb899f762b9e717f44eaf3fae7c00cd3e.

Signed-off-by: Daniel Stone <daniel.stone@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>