spi: cadence_qspi: move the sram partition in init

spi: cadence_qspi: move the sram partition in init

There is no need to re-configure sram partition for every read/write for
better full use of sram for read or write. This patch divides the half
sram for read & half for write once at initialization.

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