powerpc/reg: Add TEXASR related macros

This patches add some macros for CR0/TEXASR bits so that PR KVM TM
logic (tbegin./treclaim./tabort.) can make use of them later.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>