imx: imx7d-sdb: fix ethernet, sync .dts with linux

Commit 0d52bab4 (mx7dsabre: Enable DM_ETH) changed these flags from 0
(aka GPIO_ACTIVE_HIGH) to GPIO_ACTIVE_LOW. It claimed to "Also sync
device tree with v5.5-rc1", but in the linux tree, these gpios have
always been GPIO_ACTIVE_HIGH ever since this node was introduced
around v4.13 (linux commit 184f39b5).

I'm guessing that the reason for the GPIO_ACTIVE_LOW was to work
around the behaviour of the soft-spi driver back then, which
effectively defaulted to spi-mode 3 and not 0. That was arguably a bug
in the soft-spi driver, which then got fixed in 0e146993 (spi: add
support for all spi modes with soft spi), but that commit then broke
ethernet on this board.

Fix it by setting the gpios as active high, which as a bonus actually
brings us in sync with the .dts in the linux source tree.

Without this, one gets

    Net:   Could not get PHY for FEC0: addr 0
    No ethernet found.

With this, ethernet (at least ping and tftp) works as expected from
the U-Boot shell.

Cc: Fabio Estevam <festevam@gmail.com>
Cc: Joris Offouga <offougajoris@gmail.com>
Cc: "Christian Bräuner Sørensen" <yocto@bsorensen.net>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>