powerpc: Use SWITCH_FRAME_SIZE for prom and rtas entry

Commit 6c171994 ("powerpc/of: Remove useless register save/restore
when calling OF back") removed the saving of srr0 and srr1 when calling
into OpenFirmware. Commit e31aa453 ("powerpc: Use LOAD_REG_IMMEDIATE
only for constants on 64-bit") did the same for rtas.

This means we don't need to save the extra stack space and can use
the common SWITCH_FRAME_SIZE.

There were already no users of _SRR0 and _SRR1 so we can remove them
too.

Link: https://github.com/linuxppc/linux/issues/83


Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>