tty: amba-pl011: add .get_fifosize for ZTE device

ZTE PL011 device has a fixed FIFO size 16.  Let's add a .get_fifosize
hook for it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>