Broadcom Vulcan (ARCH_VULCAN) has been discontinued and will be deleted
soon. So, update the GPIO_XLP Kconfig entry to remove the ARCH_VULCAN
dependency.

Also update the documentation to note that Cavium ThunderX2 uses this
driver.
Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>