platform/x86: amd-pmc: fix compilation without CONFIG_RTC_SYSTOHC_DEVICE

Just hardcode the RTC to "rtc0" which is the default for
CONFIG_RTC_SYSTOHC_DEVICE and used by all standard x86 distros.

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Suggested-by: Hans de Goede <hdegoede@redhat.com>
Fixes: 59348401 ("platform/x86: amd-pmc: Add special handling for timer based S0i3 wakeup")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20211026171443.289-1-mario.limonciello@amd.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>