mtd: spi-nor: cqspi: Fix build on arches missing readsl/writesl

The x86-64 and some other architectures are missing readsl/writesl
functions, so this driver won't build on them. Use a more portable
ioread32_rep()/iowrite32_rep() instead.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alan Tull <atull@opensource.altera.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Graham Moore <grmoore@opensource.altera.com>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Yves Vandervennet <yvanderv@opensource.altera.com>
Suggested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>