MIPS: Malta: Implement get_c0_fdc_int()

MIPS: Malta: Implement get_c0_fdc_int()

Implement the weak get_c0_fdc_int() function for Malta. The Fast Debug
Channel (FDC) interrupt is obtained mainly depending on whether a GIC is
present. Vectored external interrupt mode isn't yet supported.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/9143/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>