socfpga: timer actually counts down

Timer on cyclone5 actually counts down. It took me a while to figure
out, as timer counting in wrong direction actually _can_ be used, it
just appears to tick at extremely high frequency in u-boot.

The bug was introduced in commit
23ab7ee0.

Signed-off-by: Pavel Machek <pavel@denx.de>
Acked-by: Marek Vasut <marex@denx.de>