arm64: zynqmp: Set qspi tx-buswidth to 4

All ZynqMP boards are setting up tx-buswidth to 1. Due to this the
framework only issues 1-1-1 write commands to the GQSPI driver. But the
GQSPI controller is capable of handling 1-4-4 write commands, so updated
the tx-buswidth to 4.
Using all 4 lines will increase the tx data transfer rate, as now the
tx data will be transferred on four lines instead on single line.

Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/1f1b0028106d83aa06e0777e91862a07df100fa1.1684767562.git.michal.simek@amd.com