drivers: mdio_esp32: let the REF_CLK be initialized before the PHY.

When GPIO17 or 16 is used as an external REF_CLK signal, the output is
enabled in eth_esp32.c This was added in PR number #65759 and then refined
in PR #74442. However this does not work for PHYs which need the REF_CLK
for MDIO communication, such as LAN8720A. In such cases phy_mii driver
tries to get the ID of such a PHY before REF_CLK is present. Therefore
in this PR I propose to move REF_CLK initialization from eth_esp32.c to
mdio_esp32.c which gets initialized before PHY and ETH.

Signed-off-by: Łukasz Iwaszkiewicz <lukasz.iwaszkiewicz@gmail.com>