Adds autosuspend runtime functionality to BCM UART driver.
Autosuspend is enabled at end of bcm_setup.

bcm_device_lock is used for system sleep functions as they can be
called at any time.
bcm_device_lock is not held for runtime suspend functions as this
is only enabled as long as platform device is opened.
Signed-off-by: Frederic Danis <frederic.danis@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>