serial: mxs-auart: add Alphascale ASM9260 support

Alphascale ASM9260 uart IP has some common registers with
Freescale STMP37XX. This patch provide changes which
allow to reuse mxs-auart.c code for ASM9260.

Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>