arm64: socfpga: merge Agilex and N5X into ARCH_INTEL_SOCFPGA

Agilex, N5X and Stratix 10 share all quite similar arm64 hard cores and
SoC-part.  Up to a point that N5X uses the same DTSI as Agilex.  From
the Linux kernel point of view these are flavors of the same
architecture so there is no need for three top-level arm64
architectures.  Simplify this by merging all three architectures into
ARCH_INTEL_SOCFPGA and dropping the other ARCH* arm64 Kconfig entries.

The side effect is that the INTEL_STRATIX10_SERVICE will now be
available for both 32-bit and 64-bit Intel SoCFPGA, even though it is
used only for 64-bit.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>