xtensa: beat Kconfig into shape

Instead of making support code depend on variants or platforms, the
latter should select what they need explicitely.

Otherwise this starts looking weird when support code depends on
!XTENSA_PLATFORM_FOO && !XTENSA_PLATFORM_BAR etc.

This also includes some minor fixlets like converting bool and default
to def_bool and fixing indentation and whitespace errors.

Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>