doc: i.MX7 updated doc for Zephyr release 1.12

i.MX7 updated doc sections for Zephyr release 1.12:

Architectures:
    nxp_imx/mcimx7_m4: Added support for i.MX7 Cortex M4 core

Boards:
    colibri_imx7d_m4: Added support for Toradex Colibri i.MX7 board

Drivers and Sensors:
    serial: Added support for i.MX UART interface
    gpio: Added support for i.MX GPIO

HALs:
    nxp/imx: imported i.MX7 FreeRTOS HAL

Signed-off-by: Diego Sueiro <diego.sueiro@gmail.com>