spl: fix stack usage check if gd is not initialized

Most platforms do not set up gd->start_addr_sp in SPL. Since this is
required for CONFIG_SPL_SYS_REPORT_SACK_F_USAGE to work correctly, set
up gd->start_addr_sp in SPL to the value passed to
board_init_f_init_reserve if it is not set yet.

Fixes: d8c03320 ("spl: implement stack usage check")
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>