rockchip: make sure timer7 is enabled on rk3288 platforms

timer7 supplies the architected timer and thus as has to run when
the system clocksource and clockevents drivers are registered.

While it should be the responsibility of the bootloader to do this,
and there exists a fix in a community u-boot, all u-boot based systems
that actually shipped have the mentioned issue.

Therefore to not require every developer to update their u-boot, add a
snippet for this, enabling the timer early in the kernel.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>