powerpc/32: Use r2 in wrtspr() instead of r0

wrtspr() is a function to write an arbitrary value in a special
register. It is used on 8xx to write to SPRN_NRI, SPRN_EID and
SPRN_EIE. Writing any value to one of those will play with MSR EE
and MSR RI regardless of that value.

r0 is used many places in the generated code and using r0 for
that creates an unnecessary dependency of this instruction with
preceding ones using r0 in a few places in vmlinux.

r2 is most likely the most stable register as it contains the
pointer to 'current'.

Using r2 instead of r0 avoids that unnecessary dependency.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/69f9968f4b592fefda55227f0f7430ea612cc950.1611299687.git.christophe.leroy@csgroup.eu