Merge tag 'u-boot-amlogic-20210222' of https://gitlab.denx.de/u-boot/custodians/u-boot-amlogic

- adds adc-keys button driver
- fix meson-saradc driver to get reference voltage
- add adc-keys test for sandbox
- enable adc-keys for VIM3 & VIM3L boards
- fix button.h build