ram: Move Kconfig options into their own menu entry

ram: Move Kconfig options into their own menu entry

RAM drivers using Device Model currently lack of their own Kconfig menu
entry, which makes Kconfig put all options of the class in the top-level
menu of device drivers. These options are also incorrectly grouped with
pinctrl options in the generated .config, which is hard to read. Let's
create a menu entry for these drivers.

Fixes: 6c51df6859f ("dm: Add support for RAM drivers")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Tom Rini <trini@konsulko.com>