ARM: at91: drop at91_set_serial_console

at91_set_serial_console is used to define the default console of linux.
This is already manage by the cmdline. And if the boot loader can not be
modified you can still set it by enabling the CONFIG_CMDLINE_EXTEND option.
And then the command-line arguments provided by the boot loader will be
appended to the default kernel command string.

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