rockchip: enable SARADC; add buttons hotplug and ADC kmods to default packages

rockchip: enable SARADC; add buttons hotplug and ADC kmods to default packages

Select `CONFIG_ROCKCHIP_SARADC=y` to enable Rockchip SAR ADC

Add ADC Ladder Buttons driver as a kernel module as well as `kmod-button-hotplug` to the list of
default packages for Rockchip targets that have buttons connected to ADC, not including some
devices (e.g. NanoPi R76S) that have ADC buttons which are not in the device tree.

This is needed to use buttons on Rockchip devices that are connected to ADC and not GPIO

Tested on FriendlyELEC NanoPi M5

Signed-off-by: Ryan Leung <untilscour@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/23008
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>