spi: cadence_qspi: Fix fdt read of spi-max-frequency

Fix the fdt read for spi-max-frequency as it's contained
in the child node. Current state of code is always
returning default value.

Signed-off-by: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Dinh Nguyen <dinh.linux@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Vikas Manocha <vikas.manocha@st.com>
Cc: Jagannadh Teki <jteki@openedev.com>
Cc: Pavel Machek <pavel@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>