ppc4xx: Fix APC405 build breakage

This patch fixes APC405 build, by defining CONFIG_PPC4XX_I2C. This is
needed since the move of the PPC4xx I2C driver into the drivers/i2c
directory.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Matthias Fuchs <matthias.fuchs@esd.eu>