arm64: zynqmp: Dont use 4K sector erase by default for spi-flashes

Dont use 4K sector erase by default, Disabling this
would use 64K sector erase and decreases erase time.
Also disabled by the fact that UBIFS and JFFS2 won't work
with 4K sector erase.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>