MIPS: Generic: Fix big endian CPUs on generic machine

Big endian CPUs require SWAP_IO_SPACE enabled to swap accesses to little
endian peripherals.

Without this patch, big endian kernels fail to communicate with little
endian periperals, such as PCI devices, on QEMU and FPGA based
platforms.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Fixes: eed0eabd ("MIPS: generic: Introduce generic DT-based board support")
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15105/


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