ARM: 6436/1: AT91: Fix power-saving in idle-mode on 926T processors

According to Atmel, their 926T processors (AT91 post RM9200) requires
'Wait for Interrupt' mode be entered right after disabling the processor clock
in order to minimise current consumption when idle, so do both provided we're
not running on a 920T (an RM9200).

Furthermore, get rid of the #ifndef CONFIG_DEBUG_KERNEL, since arch_idle()
can be turned off completely with the kernel parameter 'nohlt'.

Cc: Andrew Victor <avictor.za@gmail.com>
Signed-off-by: Anders Larsen <al@alarsen.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>