MIPS: Alchemy: prom_putchar is board dependent

This patch replaces the general alchemy prom_putchar() implementation
in favor of board-specific versions:  The UART where the output of
prom_putchar is directed to really depends on the board, the current
implementation hardcodes this on a per-SoC basis which is just wrong.

So a generic uart tx function is provided in the alchemy headers,
and the boards can provide their own prom_putchar with custom
destination uart, and all in-kernel alchemy boards support
early printk.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>