eSPI: Add platform specific Virtual Wire GPIOs

As per eSPI spec, there are about 64 platform specific Slave to Master
Virtual Wire GPIOs that can be used for internal usages. Currently only
few of them are used, hence added initial 8 such Virtual wire GPIOs in
the eSPI header.

Signed-off-by: Vijay Hiremath <vijay.p.hiremath@intel.com>