spi: cadence_qspi: get sram size from device tree

sram size could be different on different socs, e.g. on stv0991 it is 256 while
on altera platform it is 128. It is better to receive it from device tree.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jagannadh Teki <jteki@openedev.com>