mtd: fsl-quadspi: Access multiple chips simultaneously

Add supports for simultaneous access to multiple chips. Need to lock
the mutex before any quad spi operations and unlock the mutex after
operations complete.

Signed-off-by: Han Xu <b45815@freescale.com>
[Brian: reworked err path in fsl_qspi_prep()]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>