env: ti: boot: Use ttyS2 instead of ttyO2

env: ti: boot: Use ttyS2 instead of ttyO2

ttyO2 console enables legacy CONFIG_SERIAL_OMAP driver in kernel.
Nowadays it's preferred to use the generic CONFIG_SERIAL_8250_OMAP
driver, which being enabled via ttyS2 console. Both drivers are enabled
in multi_v7_defconfig and in omap2plus_defconfig, for compatibility
reasons. Let's switch to ttyS2 console, to be sure that standard 8250
serial driver is used.

Similar behavior can be also achieved by enabling
CONFIG_SERIAL_8250_OMAP_TTYO_FIXUP option in kernel, but it's better not
to rely on that, as it can be disabled or removed after transitional
period.

Right now on DRA7/AM57x platforms the 8250-omap driver is being probed
first, and omap-serial driver is only probed if the first one failed.
It can be seen from uart3 definition in arch/arm/boot/dts/dra7-l4.dtsi:

    compatible = "ti,dra742-uart", "ti,omap4-uart";

So the kernel already uses 8250 driver. This change basically allows
kernel developers to throw away the omap-serial driver and associated
compatibility options. Similar discussions [1,2] have started several
years ago, so it should be safe to do that now.

[1] https://patchwork.kernel.org/patch/6198471/
[2] http://processors.wiki.ti.com/index.php/Sitara_Linux_UART_-_Switching_to_8250_Driver

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Andrew F. Davis <afd@ti.com>
[trini: Update omap5_uevm]
Signed-off-by: Tom Rini <trini@konsulko.com>