MIPS: jz4740: Fix section mismatch in prom.c

This patch fixes the following section mismatch:

	WARNING: arch/mips/built-in.o(.text+0xc): Section mismatch in reference from the
	function jz4740_init_cmdline() to the variable .init.data:arcs_cmdline

While were at it, make jz4740_init_cmdline static as well.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1755/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>