kconfig: add sym_get_choice_menu() helper

Choices and their members are associated via the P_CHOICE property.

Currently, prop_get_symbol(sym_get_choice_prop()) is used to obtain
the choice of the given choice member.

We can do this without relying on P_CHOICE by checking the parent in
the menu structure.

Introduce a new helper to retrieve the choice if the given symbol is a
choice member.

This is intended to replace prop_get_symbol(sym_get_choice_prop()) and
deprecate P_CHOICE eventually.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>