README: remove description about driver model configuration options (again)

The Driver Model description in README was removed by commit
65eb659e (README: remove description about driver model
configuration options), and was revived by mistake by commit
b79dadf8 when resolving the conflict.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>