CONFIG_FPU: The architecture dependency list is redundant.
Having CPU_HAS_FPU being selected by those archs as a dependency
is sufficient and cleaner.
CONFIG_FPU_SHARING: The default should always be y to be on the safe
side here, but as a compromise for not affecting existing config, let's
move the default selection local to those configs that care, again to
avoid a growing list of conditionals here. Adjust the help text which
applies to more than just Cortex-M.
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>