ocxl: Documentation

ocxl.rst gives a quick, high-level view of opencapi.

Update ioctl-number.txt to reflect ioctl numbers being used by the
ocxl driver

Signed-off-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
[mpe: Fix up mixed whitespace as spotted by gregkh]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>