starfive: visionfive2: add device tree overlay support

device tree overlay support requires fdtoverlay_addr_r to be set

before
~~~~~~
Invalid fdtoverlay_addr_r for loading overlays

after
~~~~~
Retrieving file: /boot/overlay/rtc-ds3231.dtbo

Signed-off-by: John Clark <inindev@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>