ARM: TWD: enable one-shot mode

Allow one shot timer mode to be used with the TWD.  This allows
NOHZ mode to be used on SMP systems using the TWD localtimer.

Tested on Versatile Express and U8500.

Tested-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>