vexpress_ca9x4: Enable DM_SERIAL

This commit enables support for DM_SERIAL in the vexpress_ca9x4 boards.

When running the board with the DM_SERIAL driver, the board ran out of
memory in SPL when initialising the DM serial driver.

Thus this required an increase in the pre-allocated SRAM memory. I did
increase it to 0x800, and it now works graciously.

It could probably be set lower, but I do not see any reason not to use the
available SRAM at this point.

Also adds stdout-path to the 'chosen' node in the device tree.

Signed-off-by: Ole P. Orhagen <ole.orhagen@northern.tech>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>