tty: serial: 8250_core: allow to overwrite & export serial8250_startup()

The OMAP version of the 8250 can actually use 1:1 serial8250_startup().
However it needs to be extended by a wake up irq which should to be
requested & enabled at ->startup() time and disabled at ->shutdown() time.

v2…v3: properly copy callbacks
v1…v2: add shutdown callback

Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>