i.MX: Add i.MX7 SOC implementation.

i.MX: Add i.MX7 SOC implementation.

The following interfaces are partially or fully emulated:

    * up to 2 Cortex A9 cores (SMP works with PSCI)
    * A7 MPCORE (identical to A15 MPCORE)
    * 4 GPTs modules
    * 7 GPIO controllers
    * 2 IOMUXC controllers
    * 1 CCM module
    * 1 SVNS module
    * 1 SRC module
    * 1 GPCv2 controller
    * 4 eCSPI controllers
    * 4 I2C controllers
    * 7 i.MX UART controllers
    * 2 FlexCAN controllers
    * 2 Ethernet controllers (FEC)
    * 3 SD controllers (USDHC)
    * 4 WDT modules
    * 1 SDMA module
    * 1 GPR module
    * 2 USBMISC modules
    * 2 ADC modules
    * 1 PCIe controller

Tested to boot and work with upstream Linux (4.13+) guest.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
[PMM: folded a couple of long lines]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>