tty: amba-pl011: add helper to detect split LCRH register

Add a helper to detect the split LCRH register found on ST variants.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>