ARM: dts: dra7-evm: fix qspi device tree partition size

64KiB is allocated for qspi dtb partition which is not
sufficient, so updating the partition table size to 512KiB
for device tree partition.

This also aligns the QSPI partition definitions between
kernel and U-Boot.

Fixes: dc2dd5b8

 ("ARM: dts: dra7: Add qspi device")

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>