firmware: fw_cfg register offsets on supported architectures only

Refrain from defining default fw_cfg register offsets on
unsupported architectures -- throw an error instead. If
QEMU were to add fw_cfg support on additional architectures,
we should add them to the FW_CFG_SYSFS depends statement in
drivers/firmware/Kconfig, and provide default values for
register offsets in drivers/firmware/qemu_fw_cfg.c at that
time.

Suggested-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Gabriel Somlo <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/firmware/qemu_fw_cfg.c

···
 #  define FW_CFG_CTRL_OFF 0x00
 #  define FW_CFG_DATA_OFF 0x01
 # else
-#  warning "QEMU FW_CFG may not be available on this architecture!"
-#  define FW_CFG_CTRL_OFF 0x00
-#  define FW_CFG_DATA_OFF 0x01
+#  error "QEMU FW_CFG not available on this architecture!"
 # endif
 #endif