xilinx: zynqmp: make spi flash support optional

The set_dfu_alt_info function use the CONFIG_SYS_SPI_U_BOOT_OFFS define
to set the dfu_alt_info environment variable for qspi boot mode. Guard
the usage of CONFIG_SYS_SPI_U_BOOT_OFFS to make spi flash support
optional.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Link: https://lore.kernel.org/r/20220620163650.18756-11-stefan.herbrechtsmeier-oss@weidmueller.com


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