ARM: dts: ls1021a: fix QuadSPI-memory reg range

According to the Reference Manual, the correct size is 512 MiB.

Without this fix, probing the QSPI fails:

    fsl-quadspi 1550000.spi: ioremap failed for resource
        [mem 0x40000000-0x7fffffff]
    fsl-quadspi 1550000.spi: Freescale QuadSPI probe failed
    fsl-quadspi: probe of 1550000.spi failed with error -12

Fixes: 85f8ee78 ("ARM: dts: ls1021a: Add support for QSPI with ls1021a SoC")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>