ARM: dts: imx51: remove bogus pin definition

Pad DISPB2_SER_RS has no function DISP1_EXT_CLK.
The definition is obviusly a copy/paste error from
MX51_PAD_DISPB2_SER_RS__DISP1_PIN16.
Signed-off-by: NLothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx51-pinfunc.h

@@ -536,7 +536,6 @@
 #define MX51_PAD_DISPB2_SER_CLK__DISP1_PIN7		0x2c4 0x6c4 0x000 0x3 0x0
 #define MX51_PAD_DISPB2_SER_CLK__DISPB2_SER_CLK		0x2c4 0x6c4 0x000 0x0 0x0
 #define MX51_PAD_DISPB2_SER_CLK__GPIO3_7		0x2c4 0x6c4 0x990 0x4 0x1
-#define MX51_PAD_DISPB2_SER_RS__DISP1_EXT_CLK		0x2c8 0x6c8 0x000 0x2 0x0
 #define MX51_PAD_DISPB2_SER_RS__DISP1_PIN16		0x2c8 0x6c8 0x000 0x2 0x0
 #define MX51_PAD_DISPB2_SER_RS__DISP1_PIN8		0x2c8 0x6c8 0x000 0x3 0x0
 #define MX51_PAD_DISPB2_SER_RS__DISPB2_SER_RS		0x2c8 0x6c8 0x000 0x0 0x0