arm: ti: Increase malloc size to 16MB for armv7 boards

The current size of 1MB is not enough use to use DFU. Increase it for
ARMv7 boards, all of which should have 32MB or more SDRAM.

With this change it is possible to do 'dfu mmc 0' on a Beaglebone Black.

Signed-off-by: Simon Glass <sjg@chromium.org>