of: reserved_mem: Add functions to parse "memory-region"

Drivers with "memory-region" properties currently have to do their own
parsing of "memory-region" properties. The result is all the drivers
have similar patterns of a call to parse "memory-region" and then get
the region's address and size. As this is a standard property, it should
have common functions for drivers to use. Add new functions to count the
number of regions and retrieve the region's address as a resource.

Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20250423-dt-memory-region-v2-v2-1-2fbd6ebd3c88@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>