spi: nxp-fspi: Use new readl_poll_sleep_timeout API

Board gets reset when performing burst read/write operations. On the
other hand, no such behaviour is observed on small size operations.

In Linux, readl_poll_timeout API already adds delay of 1us which is
further skipped in U-boot. Hence, use new "readl_poll_sleep_timeout" API
which adds delay alongwith timeout functionality.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>