doc: Add OPAL_I2C_REQUEST documentation
doc: Add OPAL_I2C_REQUEST documentation

Fixes: https://github.com/open-power/skiboot/issues/132
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.ibm.com>

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