acpigen_ps2_keybd: Add assistant key to linux,keymap

If the ChromiumOS EC indicates that the device has an assistant key,
we should also add it to the generated linux,keymap binding.  This
commit simply does so by examining the keyboard capabilities reported by
the EC.

BUG=b:333088656
TEST=With a device that has an assistant key, flash AP FW and verify
that the key is mapped to `KEY_ASSISTANT` in the Linux kernel using
`evtest`.

Change-Id: I217220e89bce88e3045a4fc3b124954696276442
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81996
Reviewed-by: Jonathon Hall <jonathon.hall@puri.sm>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Caveh Jalali <caveh@chromium.org>

src/ec/google/chromeec/ec_acpi.c

@@ -257,6 +257,7 @@ static void fill_ssdt_ps2_keyboard(const struct device *dev)
 				 !!(keybd.capabilities & KEYBD_CAP_FUNCTION_KEYS),
 				 !!(keybd.capabilities & KEYBD_CAP_NUMERIC_KEYPAD),
 				 !!(keybd.capabilities & KEYBD_CAP_SCRNLOCK_KEY),
+				 !!(keybd.capabilities & KEYBD_CAP_ASSISTANT_KEY),
 				 true);
 }