phy: ti: gmii-sel: Update methods for fetching and using qsgmii main port

The number of QSGMII main ports are specific to the device. TI's J7200 for
which the QSGMII main port property is fetched from the device-tree has
only one QSGMII main port. However, devices like TI's J721e support up to
two QSGMII main ports. Thus, the existing methods for fetching and using
the QSGMII main port are not scalable.

Update the existing methods for handling the QSGMII main ports and its
associated requirements to make it scalable for future devices.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20221026074532.109220-3-s-vadapalli@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>