ARM: SMP: use more sane register allocation for __fixup_smp_on_up

Use r0,r3-r6 rather than r0,r3,r4,r6,r7, which makes it easier to
understand which registers can be modified.  Also document which
registers hold values which must be preserved.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>