board: xilinx: Add missing prototypes

board: xilinx: Add missing prototypes

Add missing prototypes to fix the below sparse warnings
1. warning: no previous prototype for 'soc_name_decode'
[-Wmissing-prototypes]
2. warning: no previous prototype for 'soc_detection'
[-Wmissing-prototypes]
3. warning: no previous prototype for 'board_name_decode'
[-Wmissing-prototypes]
4. warning: no previous prototype for 'board_detection'
[-Wmissing-prototypes]

Signed-off-by: Algapally Santosh Sagar <santoshsagar.algapally@amd.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Link: https://lore.kernel.org/r/20230523055626.14742-1-ashok.reddy.soma@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>