ARM: pxa: select I2C_GPIO only if I2C is on

The Arcom/Eurotech VIPER SBC enables the I2C_GPIO driver, but
that has a dependency on I2C, and causes build failures if I2C
is disabled. To keep existing configurations running while fixing
the randconfig problems, this changes the logic to only enable
I2C_GPIO if I2C is already enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Daniel Mack <zonque@gmail.com>