clk: socfpga: allow building N5X clocks with ARCH_N5X

The Intel's eASIC N5X (ARCH_N5X) architecture shares a lot with Agilex
(ARCH_AGILEX) so it uses the same socfpga_agilex.dtsi, with minor
changes.  Also the clock drivers are the same.

However the clock drivers won't be build without ARCH_AGILEX.  One could
assume that ARCH_N5X simply depends on ARCH_AGILEX but this was not
modeled in Kconfig.  In current stage the ARCH_N5X is simply
unbootable.

Add a separate Kconfig entry for clocks used by both ARCH_N5X and
ARCH_AGILEX so the necessary objects will be built if either of them is
selected.

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