global: Migrate CONFIG_SH_ETHER_CACHE_WRITEBACK to CFG

Perform a simple rename of CONFIG_SH_ETHER_CACHE_WRITEBACK to CFG_SH_ETHER_CACHE_WRITEBACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/README b/README
index 2d3a48e..5f688d7 100644
--- a/README
+++ b/README
@@ -547,7 +547,7 @@
 			CONFIG_SH_ETHER_PHY_ADDR
 			Define the ETH PHY's address
 
-			CONFIG_SH_ETHER_CACHE_WRITEBACK
+			CFG_SH_ETHER_CACHE_WRITEBACK
 			If this option is set, the driver enables cache flush.
 
 - TPM Support: