imx: Fix build of mx6sxsabresd

Commit 224beb833e544b802f08765271cec07667d39669 add clock
enabling function for FEC, but the masks are not available
for SX processor and the mx6sxsabresd cannot be built clean.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <fabio.estevam@freescale.com>
CC: Nikita Kiryanov <nikita@compulab.co.il>

arch/arm/cpu/armv7/mx6/clock.c

@@ -78,7 +78,7 @@ void enable_usboh3_clk(unsigned char enable)
 
 }
 
-#ifdef CONFIG_FEC_MXC
+#if defined(CONFIG_FEC_MXC) && !defined(CONFIG_MX6SX)
 void enable_enet_clk(unsigned char enable)
 {
 	u32 mask = MXC_CCM_CCGR1_ENET_CLK_ENABLE_MASK;