arm: soc: Elaborate Kconfig strings for MPU selection

Some of the SoCs specify "Enable MPU" in Kconfig for enabling MPU
support. This will create ambiguity while selecting options via
menuconfig. Hence, append SoC family name to the selection strings
to looks more elaborative.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>