MIPS: Move arch_mem_init() comment near definition

The comment describing arch_mem_init() was separated from the definition
of arch_mem_init() by commit a09fc446

 ("[MIPS] setup.c: use
early_param() for early command line parsing"). Move the comment such
that it's next to the definition again for ease of reading.

Signed-off-by: Paul Burton <paul.burton@mips.com>