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>
xtensa: make MMU-related configuration options depend on MMU

MMUv3 and HIGHMEM support are available only on configurations with MMU,
don't show them otherwise.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: sort 'select' statements in Kconfig

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: select HAVE_IDE only on platforms that may have it

HAVE_IDE is not a property of architecture but of a platform, and neither
ISS or XTFPGA support it.

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