vybrid: Define the imx_get_mac_from_fuse() as a __weak function

The proposed way of reading fused MAC in the imx_get_mac_from_fuse() may
be different for other boards.

This commit defines the imx_get_mac_from_fuse() as a weak function to allow
board file overriding it with customized function.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
diff --git a/arch/arm/cpu/armv7/vf610/generic.c b/arch/arm/cpu/armv7/vf610/generic.c
index e0c0b1b..90fa695 100644
--- a/arch/arm/cpu/armv7/vf610/generic.c
+++ b/arch/arm/cpu/armv7/vf610/generic.c
@@ -252,7 +252,7 @@
 );
 
 #ifdef CONFIG_FEC_MXC
-void imx_get_mac_from_fuse(int dev_id, unsigned char *mac)
+__weak void imx_get_mac_from_fuse(int dev_id, unsigned char *mac)
 {
 	struct ocotp_regs *ocotp = (struct ocotp_regs *)OCOTP_BASE_ADDR;
 	struct fuse_bank *bank = &ocotp->bank[4];