log: Add a category for filesystems

Sometimes it is useful to log things related to filesystems. Add a new
category and place it at the top of one of the FAT files.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/log.c b/common/log.c
index 57b71ed..7cfc49b 100644
--- a/common/log.c
+++ b/common/log.c
@@ -30,6 +30,7 @@
 	"acpi",
 	"boot",
 	"event",
+	"fs",
 };
 
 _Static_assert(ARRAY_SIZE(log_cat_name) == LOGC_COUNT - LOGC_NONE,