ARM: at91: implement the standby function for pm/cpuidle

This patch groups the self-refresh on/cpu_do_idle/self-refresh off into
a single 'standby' function.

The standby routine for rm9200 has been turned into an asm routine to have
a better control of the self refresh and to prevent a memory access when
running this code.

Draining the write buffer is done automatically when switching for the self
refresh on sam9, so the instruction is added to the rm9200 only.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>