i.MX: Add i.MX6 SOC implementation.

i.MX: Add i.MX6 SOC implementation.

For now we only support the following devices:
* up to 4 Cortex A9 cores
* A9 MPCORE (SCU, GIC, TWD)
* 5 i.MX UARTs
* 2 EPIT timers
* 1 GPT timer
* 3 I2C controllers
* 7 GPIO controllers
* 6 SDHC controllers
* 5 SPI controllers
* 1 CCM device
* 1 SRC device
* various ROM/RAM areas.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>