arm64: dts: Reduce size of virtio block device for foundation model

Will Deacon observed that kvmtool uses a size of 0x200 for virtio
block memory region and that the virtio block spec only uses 31 bytes in
the device specific region at 0x100 so reduce the region to a less
wasteful 0x200.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>