emac_rmii_clock_mode_t enum

Default values configured using Kconfig are going to be used when "Default" selected. @warning Deprecated option. Clock configuration using Kconfig is limitedly supported only for ESP32 SoC via @c ETH_ESP32_EMAC_DEFAULT_CONFIG and is going to be reevaluated in the next major release. Clock mode and clock GPIO number is supposed to be defined in `EMAC specific configuration` structure from user's code.

Input RMII Clock from external. EMAC Clock GPIO number needs to be configured when this option is selected.

Output RMII Clock from internal (A/M)PLL Clock. EMAC Clock GPIO number needs to be configured when this option is selected.