rockchip: add description for TPL_ROCKCHIP_COMMON_BOARD

SPL_ROCKCHIP_COMMON_BOARD, an almost identical option, has a title but
this one doesn't for some reason. Add a description to make the menu
easier to read.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>