Kconfig: Add CONFIG_HASH to enable hashing API

At present CONFIG_CMD_HASH enables the 'hash' command which also brings
in the hashing API. Some boards may wish to enable the API without the
command. Add a separate CONFIG to permit this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>