sim: frv: delete UART device support

sim: frv: delete UART device support

This looks like copy & paste logic from the m32r port (and history
suggests this as well).  Since building with hw & device support
enabled leads to failures:
sim/frv/devices.c: In function 'device_io_read_buffer':
sim/frv/devices.c:39:15: error: 'UART_INCHAR_ADDR' undeclared (first use in this function)

Delete it entirely.  We leave device support in place as it is used
to flush the scache.