i2c-nomadik: turn the platform driver to an amba driver

The i2c-nomadik gateware is really a PrimeCell APB device. By hosting
the driver under the amba bus we can access it more easily, for
example using the generic pci-amba driver. The patch also fixes the
mach-ux500 users, so they register an amba device instead than a
platform device.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>