Bluetooth: L2CAP_BR: Support dynamic PSM allocation

In the function `bt_l2cap_br_server_register()`, the PSM cannot be
dynamic allocated. And only pre-set PSM is supported.

Improve the function `bt_l2cap_br_server_register()` to support the
dynamic PSM allocation if the passed PSM is zero.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>