ARM: 8283/1: sa1100: collie: clear PWER register on machine init

Let kernel drivers to control wakeup sources instead of hardcoding them
in the collie.c board file.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>