tty: serial: altera_uart: Use port->regshift to store bus shift

Use the regshift member of struct uart_port to store the address stride
from platform data. This way we can save one dereference per call of
altera_uart_readl and altera_uart_writel.

This also allows us to use the driver without platform data, which is
needed for device tree support in the Nios2 port.

Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>