ARM: dts: exynos: Specify snps, dwmac in compatible string for gmac

devicetree binding for stmmac states:
- compatible: Should be "snps,dwmac-<ip_version>", "snps,dwmac"
	For backwards compatibility: "st,spear600-gmac" is also supported.

No functional change intended.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>