i2c: at91: fix unsed variable warning when building with !CONFIG_OF

Commit 70d46a24 "i2c: at91: add dt support to i2c-at91"
added DT only support for at91sam9x5. Building i2c-at91
without CONFIG_OF now warns about at91sam9x5_config as
being unused.

  drivers/i2c/busses/i2c-at91.c:556:30: warning: 'at91sam9x5_config' defined but not used [-Wunused-variable]

Move at91sam9x5_config under the defined(CONFIG_OF)
guard as new AT91 SoCs will be DT only.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>