phy: ti: gmii-sel: Allow parent to not be syscon node

If the parent node is not a syscon type, then fallback and check
if we can get a regmap from our own node. This no longer forces
us to make the parent of this node a syscon node when that might
not be appropriate.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Link: https://lore.kernel.org/r/20230713200957.134480-1-afd@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>