board: stm32mp1: Remove gpio_hog_probe_all() from board

DM_GPIO_HOG flag has been replaced by GPIO_HOG flag since a while in
commit 49b10cb4 ("gpio: fixes for gpio-hog support").

And furthermore, gpio_hog_probe_all() is already called in board_r.c.
So gpio_hog_probe() can be removed from stm32mp1.c.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>