drivers: flash: flash_mcux_flexspi_nor: fix quad enable sequence

Writing the quad enable bit on flash chips typically requires a write
enable instruction be issued before writing the non-volatile status
register, and the flash may remain busy briefly after programming this
bit. Add code to send the WREN instruction, and to wait for the flash to
finish programming after writing the status register.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>