dt-bindings: u-boot: Add variables for bootscript location

[michalsimek]

Add bootscr-address and offset-from-ram-start properties to help in easier picking of boot script file when automated flows are used.

The bootscr-address holds the full 64 bit address of the bootscript file.

The bootscr-offset-from-ram-start holds the offset address of the bootscript file from the start of the ram base in systems with RAM detection.

[michalsimek]

Failures are in installing dependencies phase. Based on discussion with @sjg20 I have renamed offset property to bootscr-ram-offset. Fixed property type and ensure that only one property is valid at a time with updating example.

[michalsimek]

@robherring: Can you please merge it?

[robherring]

The property names in the commit msg don't match the actual name.

[michalsimek]

@robherring commit message updated.