spi: cadence_qspi: Select flash subnode at runtime

Currently spi driver gets flash parameter from first subnode.

Few boards have more than one flash with different parameters
and selection of flash is done by on board switch settings.
In such case, uboot needs to be recompiled with updated
device tree to align with board switch settings.

This patch allows to select flash node at runtime.

Boards those are supporting multiple flashes
needs to implement cadence_qspi_get_subnode function and return correct
flash node.

Cc: Apurva Nandan <a-nandan@ti.com>
Signed-off-by: Udit Kumar <u-kumar1@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>