haven: dbl: Support registration before dbl probe, resource population

Currently, doorbell registration fails if a client driver attempts to
probe before the doorbell probe since data structures are not
initialized. Add an initialized flag which registration functions can
check if data structures are initialized.

Doorbell RX registration also fails if done before cap table is
populated since RX IRQ is not yet known and RX registration attempts to
register IRQ handler. IRQ registration should happen when populating the
doorbell capabilities regardless of whether a client has registered.

Change-Id: I448b291a5885683bee48b2eb34b51faba69a03bf
Signed-off-by: Elliot Berman <eberman@codeaurora.org>