watchdog: Remove WATCHDOG_RESET macro

Now that we've globally replaced all WATCHDOG_RESET occurances, let's
remove the ugly macro itself in the header.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Tom Rini <trini@konsulko.com> [am335x_evm, mx6cuboxi, rpi_3,dra7xx_evm, pine64_plus, am65x_evm, j721e_evm]