watchdog: bcm2835: Fix poweroff behaviour

Currently poweroff/halt results in a reboot on the Raspberry Pi.
The firmware uses the RSTS register to know which partiton to
boot from. The partiton value is spread into bits
0, 2, 4, 6, 8, 10. Partiton 63 is a special partition used by
the firmware to indicate halt.

The firmware made this change in 19 Aug 2013 and was matched
by the downstream commit:
Changes for new NOOBS multi partition booting from gsh

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>