plat/arm: Use common build flag for using generic sp804 driver

SP804 TIMER is not platform specific, and current code base adds
multiple defines to use this driver. Like FVP_USE_SP804_TIMER and
FVP_VE_USE_SP804_TIMER.

This patch removes platform specific build flag and adds generic
flag `USE_SP804_TIMER` to be set to 1 by platform if needed.

Change-Id: I5ab792c189885fd1b98ddd187f3a38ebdd0baba2
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index caf5990..27f8f2a 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -311,3 +311,6 @@
 
 # Build option to provide openssl directory path
 OPENSSL_DIR			:= /usr
+
+# Build option to use the SP804 timer instead of the generic one
+USE_SP804_TIMER			:= 0