watchdog: Fix SPL build with watchdog disabled in asm files

Allow to compile assembler files in SPL build which calls WATCHDOG_RESET
function when watchdog is disabled in SPL and enabled in U-Boot proper.

This issue was fixed in past by commit 7fbd42f5 ("watchdog: Handle SPL
build with watchdog disabled") for C source files, but not for assembler
source files.

Currently the only assembler source file which calls WATCHDOG_RESET is
arch/powerpc/lib/ticks.S, so this patch affects and fixes powerpc SPL
builds.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>