Fix dump_avail[] for MALTA platforms to include the kernel.

dump_avail[] is supposed to be a superset of phys_avail[] that
describes all of the memory ranges that should be included in a full
dump. minidumps don't consider pages described by dump_avail[] to be
valid and thus they are excluded via the is_dumpable() function. Most
MIPS platforms (including MALTA) set dump_avail[] to be identical to
phys_avail[]. In particular, phys_avail[] doesn't include the kernel
itself, so pages for the kernel and it's global variables are not
considered dumpable and not included in the dump. Fix this by setting
dump_avail[0] to the first memory address (0) rather than the end of
the kernel.

Several other MIPS platforms have the same bug, though I am only able
to test malta in qemu. The correct fix is to set dump_avail[] to
describe RAM and in particular to not set dump_avail[0] to the end of
the kernel (kernel_kseg0_end).

Sponsored by: DARPA / AFRL