ARM: imx: Add initial support for i.MXRT10xx family

The i.MXRT10xx family of processors features NXP's implementation of the
Arm Cortex-M7 core and in some case the Arm Cortex-M4 core too.

This patch aims to add an initial support for imxrt.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
[Jesse: removed SOC_IMXRT's 'depends on ARCH_MULTI_V7' and 'select
ARM_GIC if ARCH_MULTI_V7']
Signed-off-by: Shawn Guo <shawnguo@kernel.org>