MIPS: Alchemy: Resolve prom section mismatches

The function prom_init_cmdline() references the variable __initdata
arcs_cmdline.

The function prom_get_ethernet_addr() references the variable __initdata
arcs_cmdline.

Annotate prom_init_cmdline() as __init, unexport and annotate
prom_get_ethernet_addr() since it's no longer called from within
driver code.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/1547/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>