mips: enable _machine_restart for spl

The sysreset driver has a config CONFIG_SPL_SYSRESET for the spl stage.
Change CONFIG_SYSRESET to CONFIG_IS_ENABLED(SYSRESET) will give spl a
chance to use _machine_restart instead of the sysreset driver.

Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>