ARM: 5778/1: AT91: Add cpuidle support

This patch adds the support for cpuidle on AT91 SoCs, taken from the
cpuidle support in mach-kirkwood.
cpuidle needs sdram_selfrefresh_enable and _disable, so move their
definition to a separate header file instead of duplicating the code
already used in pm.c.

Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>