mtd: add prototypes for weak function

This patch adds a prototype for the weak function
board_mtdparts_default().

It solves one warning when compiling with W=1 on stm32mp1 board:

board/st/stm32mp1/stm32mp1.c:
     warning: no previous prototype for 'board_mtdparts_default'
              [-Wmissing-prototypes]
     void board_mtdparts_default(const char **mtdids,
                                 const char **mtdparts)
          ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>