arm: stm32mp: use correct weak function name spl_board_prepare_for_linux

arm: stm32mp: use correct weak function name spl_board_prepare_for_linux

Replace the function spl_board_prepare_for_boot_linux by the correct
name of the weak function spl_board_prepare_for_linux defined in spl.h.

This patch avoids warning with W=1 option:

u-boot/arch/arm/mach-stm32mp/spl.c:150:6:
warning: no previous prototype for ‘spl_board_prepare_for_boot_linux’
[-Wmissing-prototypes]

Fixes: dc7e5f190de5 ("arm: stm32mp: activate data cache on DDR in SPL")
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>