ARM: shmobile: ape6evm-reference: Correct BSC bus range

The address space for the r8a73a4 Bus State Controller covers the first
512 MiB, not the first 2 GiB.
Correct the size in the "ranges" property to reflect this, and to no
longer overlap with PCI Express Memory.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>