regulator: dt-bindings: dlg,da9121: allow different names

Regulator names, as specified by regulator-name, are board-dependent and
should describe the name used on the actual board.  Do not enforce
generic buck1/2 names.

This also fixes dtbs_check warnings like:

  zynqmp-sm-k26-revA.dtb: pmic@33: regulators:buck1:regulator-name:0: 'da9131_buck1' does not match '^BUCK([1-2])$'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230725124629.150113-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>