serial: imx: add wrappers for writel and readl

This prepares implementing shadow copies for the control registers and
additionally provides a good place to hook in debug code to trace
register usage.

Most of this patch was done using pattern substitution:

	perl -p -i -e '
		s/\breadl(?:_relaxed)?\((?:sport->port\.|port->)membase \+/imx_uart_readl(sport,/;
		s/\bwritel(?:_relaxed)?\(([^,]*), (sport->port\.|port->)membase \+/imx_uart_writel(sport, $1,/;
	' drivers/tty/serial/imx.c
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>