Add pciehp_set_indicators() to set power and attention indicators with a
single register write.

This is a minor optimization because we frequently set both indicators and
this can do it with a single command.  It also reduces the number of
interfaces related to the indicators and makes them more discoverable
because callers use the PCI_EXP_SLTCTL_ATTN_IND_* and
PCI_EXP_SLTCTL_PWR_IND_* definitions directly.

[bhelgaas: extend commit log, s/PCI_EXP_SLTCTL_.*_IND_NONE/INDICATOR_NOOP/
so they don't look like things defined by the spec, add function doc, mask
commands to make it obvious we only send valid commands
(pcie_do_write_cmd() does mask it, but requires more effort to verify)]
Link: https://lore.kernel.org/r/20190903111021.1559-2-efremov@linux.comSigned-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>