dts: bindings: memc: stm32-fmc: reorder parameters to match code

Description of parameters in st,control property didn't match the values
used in the code.
Modify the description to match with the current driver implementation.
Also add a description for reg property to help setting it properly and
add corresponding dt-bindings.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>