spi: xlp: update for ARCH_VULCAN2

ARCH_VULCAN arm64 platform (for Broadcom Vulcan ARM64 processors) has
been discontinued. Cavium's ThunderX2 CN99XX (ARCH_THUNDER2) will be
the next revision of the platform.

Update compile dependencies and ACPI ID to reflect this change. There
is not need to retain ARCH_VULCAN since the Vulcan processor was never
in production and ARCH_VULCAN will be deleted soon.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Mark Brown <broonie@kernel.org>