arm64: dts: rockchip: fix NanoPi R2S GMAC clock name

arm64: dts: rockchip: fix NanoPi R2S GMAC clock name

This commit fixes the name for the GMAC clock to gmac_clkin, as this is
the name of the clock provided by the rk3328-clk driver.

Without this commit, the GMAC will not work in TX direction.

Fixes: f1ec83f880db ("arm64: dts: rockchip: Add support for FriendlyARM NanoPi R2S")
Suggested-by: Tobias Waldvogel <tobias.waldvogel@gmail.com>
Signed-off-by: David Bauer <mail@david-bauer.net>
Link: https://lore.kernel.org/r/20201026162721.70672-1-mail@david-bauer.net
Signed-off-by: Heiko Stuebner <heiko@sntech.de>