add nvmxip_qspi driver under UCLASS_NVMXIP
The device associated with this driver is the parent of the blk#<id> device
nvmxip_qspi can be reused by other platforms. If the platform
has custom settings to apply before using the flash, then the platform
can provide its own parent driver belonging to UCLASS_NVMXIP and reuse
nvmxip-blk driver. The custom driver can be implemented like nvmxip_qspi in
addition to the platform custom settings.
Platforms can use multiple NVM XIP devices at the same time by defining a
DT node for each one of them.
For more details please refer to doc/develop/driver-model/nvmxip_qspi.rst
Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>