mips: mt76xx: Use correct timer frequency

Testing has shown that the timer on the MT7688 platforms does not run
correctly (too fast timeout). This patch changes
CONFIG_SYS_MIPS_TIMER_FREQ from 200MHz to 290MHz which is the correct
value, as its also used in Linux.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>