MIPS: Malta: Setup RAM regions via DT

Move memory configuration to be performed via device tree for the Malta
board. This moves more Malta specific code to malta-dtshim.c, leaving
the rest of the mti-malta code a little more board-agnostic. This will
be useful to share more code between boards, with the device tree
providing the board specifics as intended.

Since we can't rely upon Malta boards running a bootloader capable of
handling devictrees & filling in the required information, a piece of
shim code (malta_dt_shim) is added to consume the (e)memsize variables
provided as part of the bootloader environment (or on the kernel command
line) then generate the DT memory node using the provided values.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: Kumar Gala <galak@codeaurora.org>
Cc: linux-kernel@vger.kernel.org
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Patchwork: https://patchwork.linux-mips.org/patch/11222/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>