spi: spi-fsl-qspi: Reduce devm_ioremap size to 4 times AHB buffer size

Reduce devm_ioremap size to (4 * AHB_BUFER_SIZE) rather than mapping
complete QSPI-Memmory as driver is now independent of flash size.
Flash of any size can be accessed.

Issue was reported on platform where devm_ioremap failure is observed
with size > 256M.
Error log on LS1021ATWR :
 fsl-quadspi 1550000.spi: ioremap failed for resource [mem 0x40000000-0x7fffffff]
 fsl-quadspi 1550000.spi: Freescale QuadSPI probe failed
 fsl-quadspi: probe of 1550000.spi failed with error -12

This change was also suggested previously:
https://patchwork.kernel.org/patch/10508753/#22166385

Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Signed-off-by: Ashish Kumar <Ashish.kumar@nxp.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Link: https://lore.kernel.org/r/1587037399-18672-1-git-send-email-Ashish.Kumar@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>