Support 2GB of memory on Malta systems with FreeBSD/mips.

When 2GB of memory is enabled for QEMU's Malta emulation, the physical
memory ends at an address of 2^32 - 1. This causes an integer overflow
to zero when computing the upper bound of the second phys_avail[] range.
As a result, FreeBSD/mips kernels were only using the first 256MB of
RAM and ignoring the remaining 1.75GB. To work around this, truncate
the extended memory size to 2GB minus one page for 32-bit mips kernels.

Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D16027