xtensa: rework {CONFIG,PLATFORM}_DEFAULT_MEM_START

Drop PLATFORM_DEFAULT_MEM_START from the platform/hardware.h headers.
Provide definition of CONFIG_DEFAULT_MEM_START always, allow changing it
only in noMMU configurations when PLATFORM_WANT_DEFAULT_MEM is selected.
Change prompt and description so that it's clear that it controls
PAGE_OFFSET and PHYS_OFFSET.

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