xtensa: fix screen_info, clean up unused declarations in setup.c

screen_info on xtensa has old style initializer without field names.
It was broken by the commit d9b26352

 ("x86, setup: Store the boot
cursor state") that split the unused2 field into two.

Rewrite screen_info initialization with explicit field names.

Drop unused declarations/tentative definitions of RTC- and FD-related
structures. Drop inclusion of linux/timex.h when RTC is enabled in
configuration.

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