arm: mvebu: dts: mvebu-u-boot.dtsi: Add "u-boot, dm-pre-reloc" to timer DT node

Adding the "u-boot,dm-pre-reloc" DT property to the timer node is
necesssary to support the timer in the early boot phases (e.g.
SPL & pre-reloc).

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Tony Dinh <mibodhi@gmail.com>