arm64: mvebu: armada-ap806.dtsi: Add clock-frequency to UART DT node

arm64: mvebu: armada-ap806.dtsi: Add clock-frequency to UART DT node

The clock frequency needs to be provided in the DT. Otherwise the driver
won't start in U-Boot.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Neta Zur Hershkovits <neta@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Omri Itach <omrii@marvell.com>
Cc: Igal Liberman <igall@marvell.com>
Cc: Haim Boot <hayim@marvell.com>
Cc: Hanna Hawa <hannah@marvell.com>