gtimer: Add framework time driver implementation

This commit adds an implementation of the framework time driver to the
`gtimer` module. This driver needs to be instantiated by the firmware
with a `gtimer` configuration and returned to the framework through the
`fmw_time_driver` function.

Change-Id: I5f385e2cfcaf2b22c3717b0e0d7b1f5abe1990ab
Signed-off-by: Chris Kay <chris.kay@arm.com>