cbmem.h: Change return type of cbmem_get_region

The underlying IMD function already returns an integer which indicates
success or failure.

This removes the need to have initialized variables that need to be
checked for NULL later. In some cases this actually adds the appropriate
check for returned values.

Dying is appropriate if cbmem is not found as it is essential to the
bootflow.

Change-Id: Ib3e09a75380faf9f533601368993261f042422ef
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84039
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/cbmem.h

@@ -102,7 +102,7 @@ void cbmem_run_init_hooks(int is_recovery);
 /* Add the cbmem memory used to the memory map at boot. */
 void cbmem_add_bootmem(void);
 /* Return the cbmem memory used */
-void cbmem_get_region(void **baseptr, size_t *size);
+int cbmem_get_region(void **baseptr, size_t *size);
 void cbmem_list(void);
 void cbmem_add_records_to_cbtable(struct lb_header *header);