ARM: OMAP2+: AM33xx: Add missing reset status info to GFX hwmod

GFX has a reset status register (PRM_GFX.RM_GFX_RSTST),
so update the GFX hwmod data with .rstst_off and .st_shift
information.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>