MIPS: ath79: enable uart during early_prink

This patch ensures, that the pinmux register is properly setup for the
boot console UART when early_printk is enabled.

[paul.burton@mips.com:
  - s/poinmux/pinmux/
  - s/uart/UART/
  - Drop extraneous parentheses.]

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org