Remove out-dated comment in FVP PWRC code

fvp_pwr_domain_on() used to program the CPUs mailbox. This changed
with commit 804040d1 but the comment documenting this code still
refers to the mailbox programming. This patch removes this out-dated
information.

Change-Id: Ibfe2a426bdda6e71f20c83a99cb223ceca9c559c

plat/arm/board/fvp/fvp_pm.c

@@ -154,11 +154,9 @@ int fvp_pwr_domain_on(u_register_t mpidr)
 	unsigned int psysr;
 
 	/*
-	 * Ensure that we do not cancel an inflight power off request
-	 * for the target cpu. That would leave it in a zombie wfi.
-	 * Wait for it to power off, program the jump address for the
-	 * target cpu and then program the power controller to turn
-	 * that cpu on
+	 * Ensure that we do not cancel an inflight power off request for the
+	 * target cpu. That would leave it in a zombie wfi. Wait for it to power
+	 * off and then program the power controller to turn that CPU on.
 	 */
 	do {
 		psysr = fvp_pwrc_read_psysr(mpidr);