rockchip: px30: Add EVB_PX30 Kconfig help

TARGET_EVB_PX30 can be possible to use other px30 boards.

Add the help text for existing EVB, so-that the new boards
which are resuing this config option can mention their board
help text.

This would help to track which boards are using EVB_PX30 config.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>