xilinx: common: Include header file to fix warning

xilinx: common: Include header file to fix warning

Prototype is missing for board_get_usable_ram_top, which is pointed by
below sparse warning. Include init.h header file to fix this.

warning: no previous prototype for 'board_get_usable_ram_top'
[-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/20230120053617.32463-2-ashok.reddy.soma@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>