net: ksz9477: store ksz bus operations functions

net: ksz9477: store ksz bus operations functions

The ksz9477 Linux kernel driver is based on regmap API to seamlessly
communicate to switch devices connected via different buses like SPI
or I2C. The current regmap implementation in U-Boot only supports
memory-mapped registers access [1].

Until regmap API with bus support is available in U-boot, introduce
struct ksz_phy_ops to store low-level ksz bus operations (I2C for now).

[1] https://lists.denx.de/pipermail/u-boot/2018-May/329392.html

Signed-off-by: Romain Naour <romain.naour@smile.fr>