MIPS: Malta: Use all available DDR by default

Malta boards can have more than 256MB DDR available, but we have
previously only made use of up to 256MB (ie. the DDR accessible via
kseg0) by default, without the user manually specifying mem= kernel
parameters. This patch causes all available DDR, as reported by the
bootloader via the ememsize or memsize environment variables or
optionally on the command line, to be used when possible without the
user needing to manually provide the memory ranges.

Malta now has 2 subtly different memory maps which have to be taken into
account when setting this up. The original memory map (referred to by
the code as v1) has up to 2GB of DDR aliased in both the upper & lower
halves of the 32 bit physical address space, with a 256MB I/O region
obscuring 0x10000000-0x1fffffff only in the lower alias. The revised v2
memory map is flat with up to 4GB DDR starting from 0x0, and the I/O
region obscures 256MB of DDR which becomes inacessible. The memory map
in use is indicated by a register provided by the rocit2 system
controller, which is checked in order to set up the kernels memory
ranges accordingly.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14273/


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