MIPS: xburst: use common serial code

[Antony Pavlov]

This commit series moves the UART code for Ingenic SoCs
from board level to machine level. So the code
can be reused for different boards or even
different Ingenic SoCs.

[PATCH 1/2] MIPS: xburst: use common serial code
[PATCH 2/2] MIPS: rzx50: switch to common mach-xburst serial code