soc: arm: kconfig: Clean up SOC_PART_NUMBER definitions

Put a common definition of the SOC_PART_NUMBER symbol in
soc/arm/Kconfig, to make it always available for ARM SoCs. Have the
other definitions extend the base definition, without repeating the
type.

Also put the help text on just the base definition. It will show up in
the generated documentation and when looking at the symbol information
at any of the definition locations in the menuconfig (after going into
show-all mode).

Trying to get rid of unnecessary "full" symbol definitions in
Kconfig.defconfig files, to make the organization clearer. It can also
help with finding unused symbols.

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