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>