Set correct value for SYS_ID_REV_SHIFT in FVP

According to documentation, the Rev field is located at bit 28 in
the V2M sysid register.

Fixes ARM-software/tf-issues#179

Change-Id: I2abb7bdc092ccd3f41f8962dc8d8d8e44e8dfdc3
Merge pull request #134 from jcastillo-arm/jc/tf-issues/179

Set correct value for SYS_ID_REV_SHIFT in FVP