arm64: Juno: Add memory mapped timer node

Juno based boards have a memory mapped timer @ 0x2a810000. This
is disabled on r0 version of the board due to an SoC errata.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jon Medhurst <tixy@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>