MLK-12671 ARM: imx: support single soc config

MLK-12671 ARM: imx: support single soc config

Need to make sure build pass with single SOC
config, in current build for single SOC config,
if both SOC_IMX7D and SOC_IMX6SX are NOT selected,
below build error will occur, add MU module
config to fix this build issue.

LD      init/built-in.o
arch/arm/mach-imx/built-in.o: In function `busfreq_probe':
:(.text+0x5370): undefined reference to `imx_mu_lpm_ready'
arch/arm/mach-imx/built-in.o: In function `bus_freq_pm_notify':
:(.text+0x5d50): undefined reference to `imx_mu_lpm_ready'
:(.text+0x5d68): undefined reference to `imx_mu_lpm_ready'
make: *** [vmlinux] Error 1

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>