net: stmmac: dwmac-sun8i: Allow getting syscon regmap from external device

On the Allwinner R40 SoC, the "GMAC clock" register is in the CCU
address space. Using a standard syscon to access it provides no
coordination with the CCU driver for register access. Neither does
it prevent this and other drivers from accessing other, maybe critical,
clock control registers. On other SoCs, the register is in the "system
control" address space, which might also contain controls for mapping
SRAM to devices or the CPU. This hardware has the same issues.

Instead, for these types of setups, we let the device containing the
control register create a regmap tied to it. We can then get the device
from the existing syscon phandle, and retrieve the regmap with
dev_get_regmap().

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>