ath79: ag71xx: remove PHY reset

Bit 8/12 of reset controller which is marked as PHY_RESET/SWITCH_RESET
in datasheets will trigger either a reset for builtin switch or assert
an external ETH0_RESET_L/ETH1_RESET_L pin, which are usually connected
to external PHY/switch. None of them should be triggered every time an
interface is brought up in ethernet driver.

Remove PHY reset support from ag71xx and definition for them in dtsi.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>