ARM: dts: imx27-phytec-phycore-rdk: Add SDHC support

This patch adds the SHDC devicetree node for PCM970 board.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutonix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>