configs: stm32f746-disco: Migrate CONFIG_CMD_CACHE to defconfig

Remove CONFIG_CMD_CACHE from include/configs/stm32f746-disco.h
and enable it in stm32f746-disco_defconfig

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/configs/stm32f746-disco_defconfig b/configs/stm32f746-disco_defconfig
index 6f07ff1..b376e8b 100644
--- a/configs/stm32f746-disco_defconfig
+++ b/configs/stm32f746-disco_defconfig
@@ -30,6 +30,7 @@
 CONFIG_CMD_DNS=y
 CONFIG_CMD_LINK_LOCAL=y
 CONFIG_CMD_BMP=y
+CONFIG_CMD_CACHE=y
 CONFIG_CMD_TIMER=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_EXT4=y