Add support for NXP XSPI

Merge series from Haibo Chen <haibo.chen@nxp.com>:

XSPI is a flexible SPI host controller which supports up to
2 external devices (2 CS). It support Single/Dual/Quad/Octal
mode data transfer.

The difference between XSPI and Flexspi is XSPI support
multiple independent execution environments (EENVs) for HW
virtualization with some limitations. Each EENV has its own
interrupt and its own set of programming registers that exists
in a specific offset range in the XSPI memory map.
The main environment (EENV0) address space contains all of the
registers for controlling EENV0 plus all of the general XSPI
control and programming registers. The register mnemonics for
the user environments (EENV1 to EENV4) have "_SUB_n" appended
to the mnemonic for the corresponding main-environment register.

Current driver based on EENV0, which means system already give
EENV0 right to linux.

This driver use SPI memory interface of the SPI framework to issue
flash memory operations. Tested this driver with mtd_debug and
UBIFS on NXP i.MX943 EVK board which has one MT35XU512ABA spi nor
flash. NOw this driver has the following key features:
- Support up to OCT DDR mode
- Support AHB read
- Support IP read and IP write
- Support two CS