board: am335x-icev2: add ethernet phy mode detection logic

Both ethernet ports can be used as CPSW ethernet (RMII mode)
or PRU ethernet (MII mode) by setting the jumper near the port.
Read the jumper value and set the pinmux, external mux and
PHY clock accordingly.

As jumper line is overridden by PHY RX_DV pin immediately
after bootstrap (power-up/reset), we have to use GPIO edge
detection to capture the jumper line status.

As u-boot doesn't provide any infrastructure for GPIO edge
detection, we directly access the GPIO registers.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>