MLK-15047-2: ARM: imx: add IMX_SOC_IMX7

MLK-15047-2: ARM: imx: add IMX_SOC_IMX7

IMX_SOC_IMX7 is referenced in makefiles and kconfig but it is not
defined, so define it and select it for both IMX7D and IMX7ULP.

Fixes the following build errors:

arch/arm/mach-imx/built-in.o: In function `update_lpddr2_freq_smp':
platform-imx-dma.c:(.text+0xf7c): undefined reference to `imx_scu_base'
platform-imx-dma.c:(text+0xf88): undefined reference to `imx_scu_base'
arch/arm/mach-imx/built-in.o: In function `update_ddr_freq_imx_smp':
platform-imx-dma.c:(text+0x330c): undefined reference to `imx_scu_base'
platform-imx-dma.c:(text+0x3318): undefined reference to `imx_scu_base'
Makefile:969: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1

Signed-off-by: Octavian Purdila <octavian.purdila@nxp.com>