serial: mps2-uart: move to dynamic port allocation

Some designs, like MPS3, expose number of virtual serial ports which
already close or exceeds MPS2_MAX_PORTS. Increasing MPS2_MAX_PORTS
would have negative impact (in terms of memory consumption) on tiny
MPS2 platform which, in fact, has only one physically populated UART.

Start with converting existent static port array to idr. As a bonus it
make driver not to fail in case when no alias was specified in device
tree.

Note: there is no need in idr_destroy() because code doesn't unload
since ce871229 ("serial: mps2-uart: make driver explicitly non-modular")

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>