include: env: ti: ti_common: Run main_cpsw0_qsgmii_phyinit conditionally

The main_cpsw0_qsgmii_phyinit command is defined only for certain TI
SoCs which have the do_main_cpsw0_qsgmii_phyinit variable set.

Add a check to ensure that the main_cpsw0_qsgmii_phyinit command is run
only for such SoCs.

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>