dt: add helper to read 64-bit integers

Add a helper similar to of_property_read_u32() that handles 64-bit
integers.

v2/v3: constify device node and property name parameters.

Cc: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>