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/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 0acc886..ae328c7 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -725,6 +725,10 @@
   path on the host machine which is used to build certificate generation and
   firmware encryption tool.
 
+- ``USE_SP804_TIMER``: Use the SP804 timer instead of the Generic Timer for
+  functions that wait for an arbitrary time length (udelay and mdelay). The
+  default value is 0.
+
 GICv3 driver options
 --------------------