doc/opal-api : Document opal-api's related to error log
doc/opal-api : Document opal-api's related to error log

It documents opal-api's related to error log.
OPAL_ELOG_READ          71
OPAL_ELOG_WRITE         72(UNUSED)
OPAL_ELOG_ACK           73
OPAL_ELOG_RESEND        74
OPAL_ELOG_SIZE          75

Signed-off-by: Mukesh Ojha <642e2ee181c06342c4240cb5a92dcc8bb7a2d3b5@linux.vnet.ibm.com>
Reviewed-by: Vasant Hegde <a6d753843573f7bfed5075c1e875b0be8007a0ab@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

stewart-ibm/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,stewart-ibm/skiboot,mikey/skiboot,open-power/skiboot,shenki/skiboot,mikey/skiboot,open-power/skiboot,legoater/skiboot,open-power/skiboot,mikey/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,shenki/skiboot