staging: omap-thermal: standardize register nomenclature to use 'GPU'

In order to keep same nomenclature across the register definition,
this change will make all 'MM' suffixes to be named 'GPU'.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>