sb-config: Fix block register index calculation.

Block register indexes are wrongly calculated because
of hashmap hash is not ordered.

To correctly calculate offset for block registers
use eeprom_layout_get_block_register_offset().

BUG=None
TEST=None

Change-Id: Icd83a306665be635adce8fb40575168784fa3e06
Reviewed-on: https://chrome-internal-review.googlesource.com/c/chromeos/third_party/dolos/+/7108884
Reviewed-by: Udaya Kiran Ammu <udaykiran@google.com>
Tested-by: Udaya Kiran Ammu <udaykiran@google.com>
Commit-Queue: Udaya Kiran Ammu <udaykiran@google.com>