xtensa: simplify addition of new core variants

Instead of adding new Kconfig options and Makefile rules for each new
core variant provide XTENSA_VARIANT_CUSTOM variant and record variant
name in the XTENSA_VARIANT_NAME variable. Adding new core variant now
means providing directory structure under arch/xtensa/variant and
specifying correct name in kernel configuration.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>