fat: Define MAX_CLUSTSIZE using CONFIG_FS_FAT_MAX_CLUSTSIZE

Define the MAX_CLUSTSIZE to default of 65536 only if
CONFIG_FS_FAT_MAX_CLUSTSIZE is not defined.
This option has been provided to save memory in some
memory constrained cases.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Acked-by: Michal Simek <monstr@monstr.eu>
diff --git a/README b/README
index 6edb2e7..256d6ec 100644
--- a/README
+++ b/README
@@ -1637,6 +1637,12 @@
 		filesystem. Available commands are cbfsinit, cbfsinfo, cbfsls
 		and cbfsload.
 
+- FAT(File Allocation Table) filesystem cluster size:
+		CONFIG_FS_FAT_MAX_CLUSTSIZE
+
+		Define the max cluster size for fat operations else
+		a default value of 65536 will be defined.
+
 - Keyboard Support:
 		CONFIG_ISA_KEYBOARD