arm64: dts: imx8mm-verdin: Do not power down eth-phy

Currently if suspending using either freeze or memory state, the fec
driver tries to power down the phy which leads to crash of the kernel
and non-responsible kernel with the following call trace:

[   24.839889 ] Call trace:
[   24.839892 ]  phy_error+0x18/0x60
[   24.839898 ]  kszphy_handle_interrupt+0x6c/0x80
[   24.839903 ]  phy_interrupt+0x20/0x2c
[   24.839909 ]  irq_thread_fn+0x30/0xa0
[   24.839919 ]  irq_thread+0x178/0x2c0
[   24.839925 ]  kthread+0x154/0x160
[   24.839932 ]  ret_from_fork+0x10/0x20

Since there is currently no functionality in the phy subsystem to power
down phys let's just disable the feature of powering-down the ethernet
phy.

Fixes: 6a57f224 ("arm64: dts: freescale: add initial support for verdin imx8m mini")
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>