sunxi: add F1C100 (arm926ej-s) support

sunxi: add F1C100 (arm926ej-s) support

This is Allwinner's ARM926EJ-S core, which is one of its early
products, reappearing in recent compact designs. The SoC includes
32/64Mb memory in the same physical package, and has display and USB
interfaces, allowing for very small footprint boards.

The target consists of basic 6.12 support, with u-boot. Instead of creating
a separate suniv target, as both the kernel and u-boot supports enough of
this SoC by now with minimal patching, add it into sunxi as a subtarget.

Link: https://github.com/openwrt/openwrt/pull/15022
Signed-off-by: Zoltan HERPAI <[email protected]>