ARM: dts: imx6sl-evk: Add an entry for MX6SL_PAD_ECSPI1_SS0__GPIO4_IO11

In case the bootloader has incorrectly configured the ALT mode of
MX6SL_PAD_ECSPI1_SS0 pad, we end up with the following probe error:

m25p80 spi0.0: found mr25h256, expected m25p32
m25p80 spi0.0: mr25h256 (32 Kbytes)

In order to avoid this issue, add an entry for MX6SL_PAD_ECSPI1_SS0 pad, so
that kernel configures the ECSPI chip select as GPIO functionality, which
results in correct SPI NOR probe.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>