arm64: dts: juno: Fix mem-timer

The Juno's mem-timer DT node was not fully compliant with the DT binding,
which has certain expectation about child nodes and their size and
address cells values.

Use a cell size of 1, as the binding requests, and spell out the ranges
property to be binding compliant.

Link: https://lore.kernel.org/r/20200513103016.130417-8-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>