dts: versatile: Add sysregs node

The Versatile boards have the same sysregs as other ARM Ltd boards. Add
the nodes in order to enable support for 24MHz counter as sched_clock.

This is a minimal node definition as the existing sub node definition
used on VExpress has some issues raised by Linus W.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>