marvell: a8k: enable BL31 cache by default

BL31_CACHE_DISABLE flag was introduced as a work-around
for the older SoC revisions. Since it is not relevant in the
newest versions, toggle it to be disabled by default.
One can still specify it by adding 'BL31_CACHE_DISABLE=1'
string to the build command.

Change-Id: I11b52dade3ff7f8ee643b8078c6e447c45946570
Signed-off-by: Marcin Wojtas <mw@semihalf.com>

plat/marvell/a8k/common/a8k_common.mk

 #
-# Copyright (C) 2016 - 2018 Marvell International Ltd.
+# Copyright (C) 2016 - 2020 Marvell International Ltd.
 #
 # SPDX-License-Identifier:     BSD-3-Clause
 # https://spdx.org/licenses
@@ -22,7 +22,7 @@ ERRATA_A72_859971	:= 1
 MSS_SUPPORT		:= 1
 
 # Disable EL3 cache for power management
-BL31_CACHE_DISABLE	:= 1
+BL31_CACHE_DISABLE	:= 0
 $(eval $(call add_define,BL31_CACHE_DISABLE))
 
 $(eval $(call add_define,PCI_EP_SUPPORT))