cris: add dev88_defconfig

It is not possible to netboot a dev88 using etraxfs_defconfig,
since etraxfs_defconfig does not set CONFIG_ETRAX_MEM_GRP*_CONFIG
or CONFIG_ETRAX_SDRAM_GRP*_CONFIG, and the default values does not work.

This new defconfig has correct memory configuration values,
points out the correct DTB to build in (CONFIG_BUILTIN_DTB="dev88"),
enables the serial driver (CONFIG_SERIAL_ETRAXFS) and the
GPIO driver (CONFIG_GPIO_ETRAXFS), and enables LEDS.

Signed-off-by: Niklas Cassel <nks@flawful.org>
Signed-off-by: Jesper Nilsson <jespern@axis.com>