OPAL_XSCOM_READ and OPAL_XSCOM_WRITE
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These low level calls will read/write XSCOM values directly.

They should only be used by low level manufacturing/debug tools.
"Normal" host OS kernel code should not know about XSCOM.

each takes three parameters:

int xscom_read(uint32_t partid, uint64_t pcb_addr, uint64_t *val)
int xscom_write(uint32_t partid, uint64_t pcb_addr, uint64_t val)

Add basic OPAL_XSCOM_READ and OPAL_XSCOM_WRITE docs
Add basic OPAL_XSCOM_READ and OPAL_XSCOM_WRITE docs

Intentionally lazy as lazy docs are better than no docs.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>
