kconfig: Remove duplicated ARM_MPU dependency on CUSTOM_SECTION_ALIGN

CUSTOM_SECTION_ALIGN is already defined within an 'if ARM_MPU', so it
does not need a 'depends on ARM_MPU'.

Flagged by https://github.com/zephyrproject-rtos/ci-tools/pull/128

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Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>