ARM: dts: zynq: describe SLCR as simple-mfd rather than simple-bus

The SLCR (System-Level Control Registers) block is an MFD (Multi
Function Device) rather than a bus.

"simple-mfd" seems a more suitable compatible string than "simple-bus".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>