arm: dts: add missing vexpress device trees

Add the device trees for

* vexpress_ca5x2_defconfig
* vexpress_ca9x4_defconfig
* vexpress_ca15_tc2_defconfig

as available in Linux 5.1 rc5.

We are using the vexpress_ca15_tc2_defconfig and vexpress_ca9x4_defconfig
for Travis testing via QEMU.

The UEFI base Embedded Base Boot Requirements Specification (EBBR) requires
that an embedded board either provides a device tree or an ACPI table.

All block devices are meant to be moved to the driver model. On ARM this
requires a device tree.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>