Documentation/ABI: Add ABI documentation for hv-gpci format

This patch adds ABI documentation for hv-gpci event format.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201003074943.338618-3-kjain@linux.ibm.com