mips: qemu-mips/64: Expand malloc pool for CONFIG_SYS_BOOTPARAMS_LEN

Before this patch, CONFIG_SYS_BOOTPARAMS_LEN was the same size as
CONFIG_SYS_MALLOC_LEN. So, if malloc() had previously been called, and
initr_malloc_bootparams() was called, it would fail with an out-of-
memory error. This patch fixes this issue by expanding the malloc pool
to 256KB.

Signed-off-by: Kyle Edwards <kyleedwardsny@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>