dts: Add support for specifier-space property.

Currently all the *-names and *-cells properties are derived from the
name of the base <name>s property. This is a limitation because:

- It forces the base property name to be plural ending in -s
- It doesn't allow the english exception of plural words ending in -es

With this patch we add one additional property 'specifier-space' that
can be used to explicitly specify the base property name.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Suggested-by: Martí Bolívar <marti.bolivar@nordicsemi.no>