phy: xgene: rename "enum phy_mode" to "enum xgene_phy_mode"

No functional change. Rename "enum phy_mode" to
"enum xgene_phy_mode" in xgene phy driver in
preparation for adding set_mode callback in
phy core.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Loc Ho <lho@apm.com>