armsr: enable ACPI_BUTTON

A review of the generated OpenWrt kernel .config
vs the Linux arm64 defconfig showed that this
option was not being enabled, as it is disabled
in OpenWrt's generic config.

ACPI_BUTTON is needed to report and respond to
power button events, so it should be enabled.

Signed-off-by: Mathew McBride <matt@traverse.com.au>