watchdog: sp5100_tco: Use EFCH MMIO for newer Hygon FCH

watchdog: sp5100_tco: Use EFCH MMIO for newer Hygon FCH

Commit 009637de1f65 ("watchdog: sp5100_tco: support Hygon FCH/SCH
(Server Controller Hub)") added Hygon vendor matching to the efch
layout selection, but newer Hygon 0x790b SMBus devices still need the
efch_mmio path.

The efch_mmio path enables EFCH_PM_DECODEEN_WDT_TMREN before probing the
watchdog MMIO block. If firmware leaves that bit clear and the driver
picks the legacy efch path instead, probe falls back to the alternate
window and fails with "Watchdog hardware is disabled".

Select efch_mmio for Hygon 0x790b devices with revision 0x51 or later,
matching the equivalent AMD behavior and allowing the watchdog to
initialize on those systems.

Fixes: 009637de1f65 ("watchdog: sp5100_tco: support Hygon FCH/SCH (Server Controller Hub)")
Signed-off-by: Yingjie Gao <gaoyingjie@uniontech.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20260402071617.634563-2-gaoyingjie@uniontech.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>