ASoC imx-audmux: add MX31_AUDMUX_PORT7_SSI_PINS_7 define

The MX31 Audmux also has 7 Ports.
This patch adds the missing define, and makes the debugfs code iterate
over that port too.

Signed-off-by: Torben Hohn <torbenh@linutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>