[POWERPC] mpc52xx suspend to deep-sleep

Implement deep-sleep on MPC52xx.
SDRAM is put into self-refresh with help of SRAM code
(alternatives would be code in FLASH, I-cache).
Interrupt code must also not be in SDRAM, so put it
in I-cache.
MPC52xx core is static, so contents will remain intact even
with clocks turned off.

Signed-off-by: Domen Puncer <domen.puncer@telargo.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>