configs: meson64: add fdtoverlay_addr_r

In order to support loading FTD Overlays when booting with the pxe
command (or extlinux.conf), supported with [1], add the missing
fdtoverlay_addr_r used to load the overlay before applying it to
the FDT loaded at fdt_addr_r.

[1] https://patchwork.ozlabs.org/project/uboot/patch/20210120085453.2783678-1-narmstrong@baylibre.com/



Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>