soc: fsl: dpio: Change bpid type to u16

In all QBMan registers, the buffer pool id field is two bytes long.
The low level qbman APIs reflect this, but the high level DPIO ones
use u32. Modify them in order to avoid implicit downcasts.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>