spl: add g_dnl_get_board_bcd_device_number

Add g_dnl_get_board_bcd_device_number, the new BCD value is used by uuu to distinguish
if the SPL supports the SDPV.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>