env: ti: j721e-evm: Add env variable to power on & reset QSGMII PHY in J7200 EVM

MAIN CPSW0 requires the PHY to be powered on and reset for QSGMII
operation. Add a env variable to configure driving "0" on ENET_EXP_PWRDN
controlled by GPIO EXPANDER2 (I2C Addr: 0x22), PIN: 17 and driving "1"
on ENET_EXP_RESETZ controlled by GPIO EXPANDER2 (I2C Addr: 0x22),
PIN: 18.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Link: https://lore.kernel.org/r/20210721155849.20994-18-kishon@ti.com