spl: Kconfig: Add SPL dependency to CONFIG_HANDOFF

spl: Kconfig: Add SPL dependency to CONFIG_HANDOFF

CONFIG_HANDOFF is used in u-boot proper to locate handoff info from SPL
during pre-relocation init (in setup_spl_handoff). Add explicit dependency
on CONFIG_SPL, to fix the following build error when CONFIG_HANDOFF &&
!CONFIG_SPL:

common/board_f.c: In function ‘setup_spl_handoff’:
common/board_f.c:283:4: error: ‘gd_t {aka struct global_data}’
has no member named ‘spl_handoff’
  gd->spl_handoff = bloblist_find(BLOBLISTT_SPL_HANDOFF,
    ^~

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>