xilinx: zynqmp: Add the missing function prototype

Add missing prototype to fix the below sparse warning
warning: no previous prototype for 'spl_spi_get_uboot_offs'
	 [-Wmissing-prototypes]

Fixes: 2c8a0921 ("arm64: zynqmp: Add multiboot support for SPL/SPI offset calculation")
Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20240207083328.1673752-1-venkatesh.abbarapu@amd.com


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