powerpc: Include asm/reg.h in asm/svm.h

is_secure_guest() uses mfmsr().

Don't rely on users to include asm/reg.h, include
it in asm/svm.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/482c82c8a29d5fb3ea279b34f107e0e775001344.1646767214.git.christophe.leroy@csgroup.eu