serial: bcm63xx-uart: add polling support

The tty framework can support polling console to allow operation
in situations where interrupts are turned off. Adding polling mode
support allows using KGDB over serial console,

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Link: https://lore.kernel.org/r/20230322223956.84647-1-arend.vanspriel@broadcom.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>