Bluetooth: Add l2cap-register command to btshell

l2cap-register can be used to register a server:

btshell> l2cap-register 0x0080
bt: bt_l2cap_server_register (0x0011110c): PSM 0x0080

Change-Id: I2c2d9a4cba70b0cddaf352830659d206b609ee63
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>