rockchip: spl: Add common handling of USB bootsource_id 0x81

rockchip: spl: Add common handling of USB bootsource_id 0x81

Multiple newer Rockchip SoCs report 0x81 instead of 0xa when booting
from USB, i.e. on RK3576, RK3528, RK3506 and RV1106.

Move the bootsource_id 0x81 handling currently only used for RK3528 to
the common read_brom_bootsource_id() to reduce the need to override this
function when support for newer SoCs is introduced.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>