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

Commit 9f56917a ("dm: core: make simple-bus compatible to
simple-mfd") made possible to import the following commit:

Linux commit: bc5ba9b98435bf76d92e0954da1784695aa449f1

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>