MIPS: RB532: Detect uart type, add platform device

Auto-detection works just fine, so use it instead of specifying the type
manually. Also define a platform device for the uart, as suggested by
David Daney.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>