power: reset: at91-sama5d2_shdwc: fix clobber list

Assembly in at91_lpddr_poweroff has r0 in the clobber list but uses r6.

Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>