arm64: dts: ti: k3-j721s2-common-proc-board: Alias console uart to serial2

On J721s2 Linux console is on main_uart8 but to be consistent with other
J7 family of devices, alias it to ttyS2 (serial2). This also eliminates
need to have higher number of 8250 runtime UARTs.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Link: https://lore.kernel.org/r/20211223121650.26868-3-vigneshr@ti.com