MIPS: Set default CPU type for BCM47XX platforms

MIPS: Set default CPU type for BCM47XX platforms

If neither BCM47XX_SSD nor BCM47XX_BCMA is selected, then no
CPU type is available leading to build problems. We fix
this problem by using MIPS32r1 as the default CPU type for
the BCM47XX platform.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5618/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>