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/Makefile b/Makefile
index f4589d9..1b986c4 100644
--- a/Makefile
+++ b/Makefile
@@ -914,6 +914,7 @@
 $(eval $(call assert_boolean,ERRATA_SPECULATIVE_AT))
 $(eval $(call assert_boolean,RAS_TRAP_LOWER_EL_ERR_ACCESS))
 $(eval $(call assert_boolean,COT_DESC_IN_DTB))
+$(eval $(call assert_boolean,USE_SP804_TIMER))
 
 $(eval $(call assert_numeric,ARM_ARCH_MAJOR))
 $(eval $(call assert_numeric,ARM_ARCH_MINOR))
@@ -995,6 +996,7 @@
 $(eval $(call add_define,ERRATA_SPECULATIVE_AT))
 $(eval $(call add_define,RAS_TRAP_LOWER_EL_ERR_ACCESS))
 $(eval $(call add_define,COT_DESC_IN_DTB))
+$(eval $(call add_define,USE_SP804_TIMER))
 
 ifeq (${SANITIZE_UB},trap)
         $(eval $(call add_define,MONITOR_TRAPS))