arm/riscv: Remove types from SOC_SERIES in Kconfig.defconfig files

SOC_SERIES is already defined with a type in arch/Kconfig, which is
always included.

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>