drivers: flash: stm32: add "generic read/write" ex op to QSPI driver

Some external flash modules have extra commands to support, for example,
reading/writing an OTP zone. Given that the commands are highly specific
and difficult to generalize, we add two ex ops that can be used to
transmit a custom command (in the form of a full QSPI_CommandTypeDef) and
then read or write a user-provided buffer.

Signed-off-by: Federico Di Gregorio <fog@dndg.it>