net: stmmac: merge ARCH_SOCFPGA and ARCH_STRATIX10

Simplify 32-bit and 64-bit Intel SoCFPGA Kconfig options by having only
one for both of them.  This the common practice for other platforms.
Additionally, the ARCH_SOCFPGA is too generic as SoCFPGA designs come
from multiple vendors.

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