doc: thead: lpi4a: Update for S-Mode proper U-Boot support

doc: thead: lpi4a: Update for S-Mode proper U-Boot support

Proper U-Boot for Lichee Pi 4A now runs in S mode instead of M mode,
which means the extra firmware, OpenSBI, must be built and integrated
in the image, and the vendor U-Boot cannot chainload the result image
anymore as it runs in M mode.

Remove redundant information about chainloading and update build steps
to mention OpenSBI firmware.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>