cpulib: Add ISBs or comment why they are unneeded

Change-Id: I18a41bb9fedda635c3c002a7f112578808410ef6
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>

lib/cpus/aarch64/cortex_a57.S

@@ -404,6 +404,7 @@ func cortex_a57_reset_func
 #if IMAGE_BL31 && WORKAROUND_CVE_2017_5715
 	adr	x0, wa_cve_2017_5715_mmu_vbar
 	msr	vbar_el3, x0
+	/* isb will be performed before returning from this function */
 #endif
 
 #if WORKAROUND_CVE_2018_3639

lib/cpus/aarch64/cortex_a72.S

@@ -137,6 +137,7 @@ func cortex_a72_reset_func
 	cpu_check_csv2	x0, 1f
 	adr	x0, wa_cve_2017_5715_mmu_vbar
 	msr	vbar_el3, x0
+	/* isb will be performed before returning from this function */
 1:
 #endif
 

lib/cpus/aarch64/cortex_a73.S

@@ -40,6 +40,7 @@ func cortex_a73_reset_func
 	cpu_check_csv2	x0, 1f
 	adr	x0, wa_cve_2017_5715_bpiall_vbar
 	msr	vbar_el3, x0
+	/* isb will be performed before returning from this function */
 1:
 #endif
 

lib/cpus/aarch64/cortex_a75.S

@@ -15,6 +15,7 @@ func cortex_a75_reset_func
 	cpu_check_csv2	x0, 1f
 	adr	x0, wa_cve_2017_5715_bpiall_vbar
 	msr	vbar_el3, x0
+	isb
 1:
 #endif