Remove unused 'CPU present' flag

This patch removes the 'CPU present' flag that's being set but not
referred or used anywhere else.

Change-Id: Iaf82bdb354134e0b33af16c7ba88eb2259b2682a
Move architecture timer setup to platform-specific code

At present, bl1_arch_setup() and bl31_arch_setup() program the counter
frequency using a value from the memory mapped generic timer. The
generic timer however is not necessarily present on all ARM systems
(although it is architected to be present on all server systems).

This patch moves the timer setup to platform-specific code and updates
the relevant documentation. Also, CNTR.FCREQ is set as the specification
requires the bit corresponding to the counter's frequency to be set when
enabling. Since we intend to use the base frequency, set bit 8.

Fixes ARM-software/tf-issues#24

Change-Id: I32c52cf882253e01f49056f47c58c23e6f422652