i2c, avr32: fix compiler warning "input is not relaxable"

compiling U-Boot for avr32 boards shows since
commit 3d1957f0 "dm: i2c: Add support for multiplexed I2C buses"
this warning:

Building current source for 4 boards (4 threads, 8 jobs per thread)
     avr32:  +   atstk1002
+(atstk1002) drivers/i2c/built-in.o: warning: input is not relaxable
     avr32:  +   grasshopper
+(grasshopper) drivers/i2c/built-in.o: warning: input is not relaxable
     avr32:  +   atngw100
+(atngw100) drivers/i2c/built-in.o: warning: input is not relaxable
     avr32:  +   atngw100mkii
+(atngw100mkii) drivers/i2c/built-in.o: warning: input is not relaxable
    0    4    0 /4      0:00:16  : atngw100mkii

Fix it.
Signed-off-by: NHeiko Schocher <hs@denx.de>
Reviewed-by: NRoger Meier <r.meier@siemens.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NAndreas Bießmann <andreas.devel@googlemail.com>

drivers/i2c/Makefile

@@ -40,4 +40,4 @@ obj-$(CONFIG_SYS_I2C_UNIPHIER) += i2c-uniphier.o
 obj-$(CONFIG_SYS_I2C_UNIPHIER_F) += i2c-uniphier-f.o
 obj-$(CONFIG_SYS_I2C_ZYNQ) += zynq_i2c.o
 
-obj-y += muxes/
+obj-$(CONFIG_I2C_MUX) += muxes/