Implementation for 16GB version

[Kreyren]

The 16GB version of LicheePi 4A requires the following changes to work correctly:

https://github.com/ryan4yin/nixos-licheepi4a/blob/main/pkgs/u-boot/default.nix#L22

Requires:

# https://github.com/chainsx/thead-u-boot/blob/extlinux/configs/light_lpi4a_16g_defconfig
defconfig = "light_lpi4a_16g_defconfig";

https://github.com/ryan4yin/nixos-licheepi4a/blob/main/modules/licheepi4a.nix#L34

Requires:

# https://github.com/revyos/thead-kernel/blob/lpi4a/arch/riscv/boot/dts/thead/light-lpi4a-16gb.dts
name = "thead/light-lpi4a-16gb.dtb";

I figured out the stupid way that using the 8GB version will allow the device to boot, but it will be unable to use the RAM and seems to have various issues associated with using the wrong configurations

Proposing to implement a handling that allows producing u-boot and DTS for the 16GB version