ARM: imx: fix imx8m dependencies

Selecting ARM_GIC_V3 on non-CP15 processors leads to build failures
like

arch/arm/include/asm/arch_gicv3.h: In function 'write_ICC_AP1R3_EL1':
arch/arm/include/asm/arch_gicv3.h:36:40: error: 'c12' undeclared (first use in this function)
   36 | #define __ICC_AP1Rx(x)   __ACCESS_CP15(c12, 0, c9, x)
      |                                        ^~~

Add a dependency to only enable the gic driver when building for
at an ARMv7 target, which is the closes approximation to the ARMv8
processor that is actually in this chip.

Fixes: fc40200e

 ("soc: imx: increase build coverage for imx8m soc driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>