lantiq: fritz7362sl: add partition subnode for SPI flash

Without a partition subnode ofpart_core still parses direct subnodes as
partitions, but it ignores nodes with a compatible property. Due to
this, the switch to nvmem-cells made the urlader partition inaccessible.
As a result, the wireless network was broken, as the calibration data
is read from that partition by a script.

Fixes: #8983
Signed-off-by: Jan Hoffmann <jan@3e8.eu>