frv: add io{read,write}{16,32}be functions

These functions are used in various drivers, including the latest
version of the 8250 driver. The latter causes the following build
failure.

drivers/tty/serial/8250/8250_core.c: In function 'mem32be_serial_out':
drivers/tty/serial/8250/8250_core.c:456:2: error:
			implicit declaration of function 'iowrite32be'
drivers/tty/serial/8250/8250_core.c: In function 'mem32be_serial_in':
drivers/tty/serial/8250/8250_core.c:462:2: error:
			implicit declaration of function 'ioread32be'

Cc: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: c627f2ce ("serial: 8250: Add support for big-endian MMIO
	accesses")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>