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>
Tested-by: Tom Rini <trini@konsulko.com> [am335x_evm, mx6cuboxi, rpi_3,dra7xx_evm, pine64_plus, am65x_evm, j721e_evm]