enum boot_mode is defined twice once in spl.h and also in
spl_boot.c, hence removing the same from spl_boot.c and including
the header file.

Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>