op-ipmi-oem: Improve description

Reword the description and summary to more accurately reflect
what the recipe is - a plugin to the Phosphor IPMI framework
that handles OpenPOWER OEM commands.

Change-Id: I196b1d20a8c6a39880de93b844fa0884e6a1b9c1
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>