ARM: at91: do not pin mux the UARTs in init_early

There is no need to pinmux the UART so early in the kernel.
Move it to the board init.

This will also allow to finally move the gpio driver to platform device/driver.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>