soc: arm: atmel_sam: samv71: Remove ARM_MPU selection

The SAM V71 SoC configuration currently selects the `ARM_MPU` symbol
and this effectively forces MPU usage on the SoC.

This commit removes `ARM_MPU` selection from the SoC Kconfig since it
is intended to be selected by a board, and the `CPU_HAS_ARM_MPU` symbol
already indicates that the SoC supports ARM MPU.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>