rockchip: timer: fix U_BOOT_DRIVER name

When I originally added this driver, I did some careless (and in
retrospect: mindless) copy & paste for the U_BOOT_DRIVER structure
skeletion... unfortunately, the 'arc_timer' string was committed
and slipped through all reviews.

This fixes the U_BOOT_DRIVER name to read 'rockchip_rk3368_timer'
(as originally intended).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reported-by: Artturi Alm <artturi.alm@gmail.com>