button: qcom-pmic: prettify and standardise button labels

button: qcom-pmic: prettify and standardise button labels

Boards using gpio-keys for volume buttons label them "Volume Down",
let's match that here, and make the power button nicer too.

This simplifies configuring button_cmds in a board-agnostic way.

Tested-by: Danila Tikhonov <danila@jiaxyga.com> # google-sunfish
Tested-by: Jens Reidel <adrian@mainlining.org> # xiaomi-davinci
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250331-qcom-phones-v4-5-f52e57d3b8c6@linaro.org
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>