ARM: 5955/1: ep93xx: move timer defines into core.c and document

The timer defines are only used in core.c. Move them so
they will not be globaly exposed.

While here, add additional defines to document the magic
numbers used in the registers. Also, add some comments
for clarification.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>