ARM: dts: imx7d: Specify viewport count for PCIE block

i.MX7D comes with 4 viewports, so configure PCIE node accordingly so
that the driver won't assume we only have 2.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Richard Zhu <hongxing.zhu@nxp.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>