Revert "Partially Revert "dtschema: add nanoamp unit""

A patch has been sent upstream to remove the $ref from -nanoamp
properties.

This reverts commit f5f03f6.

Signed-off-by: Cosmin Tanislav <demonsingur@gmail.com>

dtschema/meta-schemas/core.yaml

@@ -58,7 +58,7 @@ definitions:
         propertyNames:
           enum: [ description, deprecated ]
 
-      '-(bits|-kBps|percent|bp|mhz|hz|sec|ms|us|ns|ps|mm|microamp(-hours)?|(micro-)?ohms|microwatt-hours|microvolt|(femto|pico)farads|(milli)?celsius|kelvin|kpascal)$':
+      '-(bits|-kBps|percent|bp|mhz|hz|sec|ms|us|ns|ps|mm|nanoamp|microamp(-hours)?|(micro-)?ohms|microwatt-hours|microvolt|(femto|pico)farads|(milli)?celsius|kelvin|kpascal)$':
         allOf:
           - $ref: cell.yaml#/array
           - description: Standard unit suffix properties don't need a type $ref