uboot-imx

sbc8641d: increase malloc pool size to a sane default

Currently the board fails to save its env, since the env size
is much smaller than the sector size, and the malloc fails for
the pad buffer, giving the user visible symptom of:

Unable to save the rest of sector (253952)

Allow for 1M malloc pool, the same as used on the sbc8548 board.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>