phy: ti: gmii-sel: retrieve ports number and base offset from dt

phy: ti: gmii-sel: retrieve ports number and base offset from dt

On K3 AM654x/J721E platforms the Port MII mode selection register(s) have
similar format and placed in the System Control Module (SCM) module
sequentially as one register per port, but, depending SOC and CPSW
instance, the base offset and number of ports can be different.

Hence, add possibility to retrieve number of ports and base registers
offset from DT and support for max possible number of ports supported by K3
SoCs like J721E.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Link: https://lore.kernel.org/r/20200828201943.29155-4-grygorii.strashko@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>