soc: xilinx: zynqmp: Mark soc_xilinx_zynqmp_get_machine() as static

Fix the following sparse and compile time warning triggered with W=1:

drivers/soc/soc_xilinx_zynqmp.c:288:5:
warning: no previous prototype for 'soc_xilinx_zynqmp_get_machine'
[-Wmissing-prototypes]

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20221004055201.26146-1-venkatesh.abbarapu@amd.com


Signed-off-by: Michal Simek <michal.simek@amd.com>