shields: mikroe_eth_click config should be made conditional

[erwango]

Describe the bug
mikroe_eth_click provides a static configuration.
This kind of configuration used to create portability issues in application (cf #20934)
It should be reworked to be conditional (.defconfig), as done in other shields

Something like (pseudo code):

îf SHIELD_MIKROE_ETH_CLICK

if NET
CONFIG_SPI=y
CONFIG_NET_L2_ETHERNET=y

# ENC28J60 Ethernet Device
CONFIG_ETH_ENC28J60=y
CONFIG_ETH_ENC28J60_0=y

endif
endif

[erwango]

@agansari sorry for late feedback, but I didn't think this would go in 2.3