MIPS: Introduce plat_get_fdt a platform API to retrieve the FDT

Early access to the kernel command line requires early access to the FDT
for platforms which pass the command line within the device tree. There
was no common way to retrieve the location of the FDT without incurring
side effects, such as plat_mem_setup which, on Malta at least,
initializes a bunch of other stuff.

This patch adds plat_get_ftd() for IMG platforms.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: kernel-hardening@lists.openwall.com
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12988/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>