board: ti: j721e: evm.c: Fix the probing of in Sierra SerDes0

Initialization and power on operations of links have been moved under the
link device in the Sierra SerDes driver. Also, the UCLASS of
sierra_phy_provider has been changed to UCLASS_MISC.

Therefore, fix the probing of SerDes0 instance accordingly.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Georgi Vlaev <g-vlaev@ti.com>