netsec: ignore 'phy-mode' device property on ACPI systems

Since commit bbc4d71d ("net: phy: realtek: fix rtl8211e rx/tx
delay config"), the Realtek PHY driver will override any TX/RX delay
set by hardware straps if the phy-mode device property does not match.

This is causing problems on SynQuacer based platforms (the only SoC
that incorporates the netsec hardware), since many were built with
this Realtek PHY, and shipped with firmware that defines the phy-mode
as 'rgmii', even though the PHY is configured for TX and RX delay using
pull-ups.

From the driver's perspective, we should not make any assumptions in
the general case that the PHY hardware does not require any initial
configuration. However, the situation is slightly different for ACPI
boot, since it implies rich firmware with AML abstractions to handle
hardware details that are not exposed to the OS. So in the ACPI case,
it is reasonable to assume that the PHY comes up in the right mode,
regardless of whether the mode is set by straps, by boot time firmware
or by AML executed by the ACPI interpreter.

So let's ignore the 'phy-mode' device property when probing the netsec
driver in ACPI mode, and hardcode the mode to PHY_INTERFACE_MODE_NA,
which should work with any PHY provided that it is configured by the
time the driver attaches to it. While at it, document that omitting
the mode is permitted for DT probing as well, by setting the phy-mode
DT property to the empty string.

Fixes: 533dd11a

 ("net: socionext: Add Synquacer NetSec driver")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201018163625.2392-1-ardb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>