riscv: bootstage: correct bootstage_report guard

riscv: bootstage: correct bootstage_report guard

Below warning can be occurred when CONFIG_BOOTSTAGE and
!CONFIG_SPL_BOOTSTAGE. It should be guarded by using CONFIG_IS_ENABLED
for SPL build.

arch/riscv/lib/bootm.c:46:9: warning: implicit declaration of
function 'bootstage_report'
   46 |         bootstage_report();
      |         ^~~~~~~~~~~~~~~~
      |         bootstage_error

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>