firmware: google: Probe for a GSMI handler in firmware

Currently this driver is loaded if the DMI string matches coreboot
and has a proper smi_command in the ACPI FADT table, but a GSMI handler in
SMM is an optional feature in coreboot.

So probe for a SMM GSMI handler before initializing the driver.
If the smihandler leaves the calling argument in %eax in the SMM save state
untouched that generally means the is no handler for GSMI.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Link: https://lore.kernel.org/r/20191118101934.22526-4-patrick.rudolph@9elements.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>