platform/x86/amd: pmc: Get STB DRAM size from PMFW

Recent PMFW's have support for querying the STB DRAM size. Add this
support to the driver.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Co-developed-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Sanket Goswami <Sanket.Goswami@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20230525141929.866385-3-Shyam-sundar.S-k@amd.com


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