spi: cadence: Add driver for xSPI

This patch ports the Cadence xSPI controller driver from
the Linux kernel. The controller supports three operating modes:

1. ACMD (Auto Command) mode
   - Includes PIO and CDMA submodes.
   - CDMA mode uses linked descriptors for high-performance,
     low-overhead operation.
   - PIO mode is suitable for simple, single-command transactions.

2. STIG (Software Triggered Instruction Generator) mode
   - Issues low-level 128-bit instructions to memory.
   - Uses the Slave DMA interface for data transfers.

3. Direct mode
   - Enables direct data access through the slave interface
     without commands.

Currently, only the STIG work mode is enabled. Additional modes will be
supported in future updates.

At the same time, also enabling the kconfig option for xSPI driver.

This driver has been ported and functionally verified on the Intel Simics
platform. It is intended for evaluation and experimental use at this stage.

Link: https://lore.kernel.org/all/[email protected]/

Signed-off-by: Boon Khai Ng <[email protected]>
Reviewed-by: Tien Fong Chee <[email protected]>

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