MIPS: PNX8550: Fix section mismatch

Triggered by pnx8550-jbs_defconfig and pnx8550-stb810_defconfig:

WARNING: vmlinux.o(.text+0xc0c): Section mismatch in reference from the function prom_getcmdline() to the variable .init.data:arcs_cmdline
The function prom_getcmdline() references
the variable __initdata arcs_cmdline.
This is often because prom_getcmdline lacks a __initdata
annotation or the annotation of arcs_cmdline is wrong.

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