ARM: socfpga_defconfig: Enable HIGHMEM

All of the SoCFPGA boards have at least 1GB of RAM, so enabling HIGHMEM
is necessary to avoid the following warning:

[    0.000000] Truncating RAM at 0x00000000-0x40000000 to -0x30000000
[    0.000000] Consider using a HIGHMEM enabled kernel.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>