ARM: socfpga: dts: Add gate-clock bindings

Add bindings for "socfpga-gate-clk" clocks. These clocks directly feed
the peripherals.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Reviewed-by: Pavel Machek <pavel@denx.de>
CC: Arnd Bergmann <arnd@arndb.de>
CC: Olof Johansson <olof@lixom.net>
Cc: Pavel Machek <pavel@denx.de>
CC: <linux@arm.linux.org.uk>
Signed-off-by: Olof Johansson <olof@lixom.net>