arm/riscv: kconfig: Remove type from NUM_IRQS in defconfig files

Add a common definition for NUM_IRQS in arch/arm/core/Kconfig and
arch/riscv/Kconfig. That way, the type doesn't have to be given for
NUM_IRQS in all the Kconfig.defconfig files.

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>