watchdog: Get rid of ASSEMBLY hacks

Only one occurance of WATCHDOG_RESET is left in one assembler file.
This patch changes this occurance to a direct call to watchdog_reset
and then removes all the ASSEMBLY ifdef'ery in watchdog.h, as it's not
needed any more to clean this mess a bit up.

Signed-off-by: Stefan Roese <sr@denx.de>