fix(gpt): correct the GPC enable sequence

Since GPC control register fields are permitted to be cached in a TLB,
invalidate TLB after setting fields to ensure future checks are using
the updated values.

Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Change-Id: I95630b40b673363bbf74da2705deca03089fff3a