mips: mt76xx: linkit-smart-7688: Misc updates to dts/config/defconfig

These misc updates include the following changes:
- Change baudrate from 57600 to 115200
- Enable MIPS_BOOT_CMDLINE_LEGACY
- Enable FIT support
- Enable ethernet support
- Enable SPI support
- Enable GPIO support
- Change max image size from 0x40000 to 0x80000

A note about the baudrate change:

The original Mediatek U-Boot version used 57600 baud. Lets move to a
more common and faster speed of 115200 baud. And remove the "console="
property from the DT as its not needed.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>