drivers: espi: xec: Enable OOB channel by default

Enable ESPI OOB channel by default in XEC driver.
Enable OOB channel transmit interrupt and handle OOB up/down correctly.
Change interrupt clearing, clear low level interrupt bits in subhandlers
and high level interrupt in aggregate handlers at the end.

Signed-off-by: Jose Alberto Meza <jose.a.meza.arellano@intel.com>