configs: stm32f746-disco: Fix ramdisk_addr_r

Set ramdisk_addr_r to 0xC0600000.

Reported-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Patrick Delaunay <patrick.delaunay@st.com>