docs/system: arm: Add sabrelite board description

This adds the target guide for SABRE Lite board, and documents how
to boot a Linux kernel and U-Boot bootloader.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20210106063504.10841-5-bmeng.cn@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>