powerpc: Add defintion for MSR[GS] to list of MSR bits

Add macros for the GS (guest state) bit to the list of MSR bit definitions.
On PowerPC cores that support embedded hypervisor mode, GS is cleared if
the system is running in hypervisor state (and MSR[PR] is cleared), and set
if it's running in guest state.  See the Power ISA 2.06 specification for
more information.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>