boards: mec15xxevb_assy6853/mec15xxevb_assy6853: Fix compile warning

If CONFIG_I2C=n is set we get a build warning:

pinmux.c:35:13: error: 'i2c_pinmux' defined but not used

Fix this by adding ifdef protection around i2c_pinmux.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>