Revert "net: cadence: macb: sama7g5_emac: Remove USARIO CLKEN flag"

Revert "net: cadence: macb: sama7g5_emac: Remove USARIO CLKEN flag"

This reverts commit db400061b5e7cc55f9b4dd15443e9838964119ea.

This commit can cause a Devicetree ABI break for older DTS files that rely this
flag for RMII configuration. Adding this back in ensures that the older
DTBs will not break.

Fixes: db400061b5e7 ("net: cadence: macb: sama7g5_emac: Remove USARIO CLKEN flag")
Signed-off-by: Ryan Wanner <Ryan.Wanner@microchip.com>
Link: https://patch.msgid.link/20250819163236.100680-1-Ryan.Wanner@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>