powerpc/vdso32: use LOAD_REG_IMMEDIATE()

Use LOAD_REG_IMMEDIATE() to load registers with immediate value.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/36f111437e66e601929308f5d5dce230e1ce472f.1575273217.git.christophe.leroy@c-s.fr