Since upstream QEMU commit 14ec3cbd7c1e ("device_tree: Increase
FDT_MAX_SIZE to 1 MiB"), which is included in release v2.12.1
and later, BL2 initialization fails with the following error (-3 is
-FDT_ERR_NOSPACE):

 ERROR:   Invalid Device Tree at 0x40000000: error -3

Increase PLAT_QEMU_DT_MAX_SIZE accordingly.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>