net: stmmac: socfpga: Use shared System Manager driver

The ARM64 System Manager requires a different method of reading
the System Manager than ARM32. A new System Manager driver was
created to steer ARM32 System Manager calls to regmap_mmio and
ARM64 System Manager calls to the new access method.

Convert from syscon to the shared System Manager driver so that
both ARM64 and ARM32 are supported.

Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>