usb: gadget: f_sdp: Add missing spl_board_prepare_for_boot() call

The spl_board_prepare_for_boot() should be called before jump_to_image_no_args()
to perform board-specific deinitialization before jumping to the next stage.
This board-specific deinitialization can be very much anything, e.g. disable
dcache in case it was enabled, or such.

Add the missing spl_board_prepare_for_boot() call into f_sdp .

Signed-off-by: Marek Vasut <marex@denx.de>