devicetree: clean up some code inconsistencies

The documentation is generally using 'inst' consistently to refer to
instance numbers, but a few helper macros have gone their own way.
Clean this up to restore consistency.

This addresses changes introduced somewhere around the following
commits:

- 4c8ed7dd9a1 ("devicetree.h: Rework
  DT_ANY_INST_HAS_PROP_STATUS_OKAY")

- ca6645d508a ("devicetree: shorten DT_ANY_INST_HAS_*_STATUS_OKAY")

- 75ab4d5507c ("devicetree: add DT_ALL_INST_HAS_PROP_STATUS_OKAY()
  macro")

- 35a8e37ac29 ("devicetree: add DT_ALL_INST_HAS_BOOL_STATUS_OKAY()
  macro")

Signed-off-by: Martí Bolívar <mbolivar@amperecomputing.com>