spi: cadence-xspi: Add COMPILE_TEST support

spi: cadence-xspi: Add COMPILE_TEST support

The Cadence XSPI driver uses readq() and writeq(), which are not provided
directly by all 32-bit architectures. Include the generic non-atomic 64-bit
I/O accessor fallback for non-64-bit builds so the driver can build there.

Drop the 64BIT dependency at the same time. The driver only needs MMIO
and the SPI memory interface at build time, and the fallback accessors
cover the 32-bit compile-test case.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260519005614.628437-1-rosenp@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>