MIPS: Fix build error due to multiple prom_putchar() definitions.

This can happen if both the generic 8250 and another early console
driver are enable.  Fixed by using an auxilliary kconfig symbol to
restrict that choice.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --cc arch/mips/Kconfig
Simple merge

diff --cc arch/mips/Kconfig.debug
Simple merge