MIPS: BCM47XX: Initialize bcma bus later (with mm available)

Initializaion with memory allocator available will be much simpler, this
will allow cleanup in the bcma code.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8234/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>