firmware: qemu config needs I/O ports

Not all machines have PCI style I/O port memory, or they do not allow
mapping it using the ioport_map() function, whcih results in a
build error with the newly added qemu firmware code:

drivers/firmware/built-in.o: In function `fw_cfg_io_cleanup':
qemu_fw_cfg.c:(.text+0x144): undefined reference to `ioport_unmap'
drivers/firmware/built-in.o: In function `fw_cfg_sysfs_probe':
qemu_fw_cfg.c:(.text+0xb18): undefined reference to `ioport_map'

This adds a Kconfig dependency to ensure the driver can only
be built on platforms that support it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 75f3e8e4 ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>