dts: pinctrl: Provide IMX_PAD_SION definition for imx53 pinctrl

The SION pin must be set for proper operation of I2C when DM is enabled.

When legacy I2C is used, this bit is set implicitly in the u-boot code:
arch/arm/include/asm/arch-mx5/iomux-mx53.h:92:
MX53_PAD_KEY_ROW3__I2C2_SDA = IOMUX_PAD(0x368, 0x040, 4 |
	IOMUX_CONFIG_SION, 0x820, 0, NO_PAD_CTRL),

The Linux kernel uses similar approach with:
arch/arm/boot/dts/imx53-tqma53.dtsi:182:
	MX53_PAD_KEY_ROW3__I2C2_SDA             0xc0000000

After applying this patch it is possible to have the I2C working with DM
on imx53 devices:

MX53_PAD_KEY_ROW3__I2C2_SDA (0x1ee | IMX_PAD_SION)
Signed-off-by: NLukasz Majewski <lukma@denx.de>

arch/arm/dts/imx53-pinfunc.h

@@ -10,6 +10,7 @@
 #ifndef __DTS_IMX53_PINFUNC_H
 #define __DTS_IMX53_PINFUNC_H
 
+#define IMX_PAD_SION       0x40000000
 /*
  * The pin function ID is a tuple of
  * <mux_reg conf_reg input_reg mux_mode input_val>