drivers/elog: use region_device for mirroring into ram

A region_device can be used to represent the in-memory mirror
of the event log. The region_device infrastructure has builtin
bounds checking so there's no need to duplicate that. In addition,
it allows for removing much of the math juggling for the buffer
size, etc.

BUG=chrome-os-partner:55932

Change-Id: Ic7fe9466019640b449257c5905ed919ac522bb58
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16097
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/elog/elog_internal.h

@@ -61,9 +61,4 @@ typedef enum elog_event_buffer_state {
 	ELOG_EVENT_BUFFER_CORRUPTED,
 } elog_event_buffer_state;
 
-struct elog_area {
-	struct elog_header header;
-	u8 data[0];
-} __attribute__((packed));
-
 #endif /* ELOG_INTERNAL_H_ */