arm/ixp4xx: Rename FREQ macro to avoid collisions

FREQ is a ridiculously short name for a platform-specific macro in a
generic header, and it now conflicts with an enumeration in the
gspca/ov519 driver.

Also delete conditional reference to ixp4xx_get_board_tick_rate()
which is not defined anywhere.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>