spi: stm32_qspi: Align reg-names with kernel 4.12 RC1

Align qspi bindings following kernel dt-bindings
Documentation/devicetree/bindings/mtd/stm32-quadspi.txt
from kernel v4.12-rc1.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>