arm: socfpga: disable CONFIG_SPL_WDT for gen5 and a10

These boards don't have a watchdog enabled in SPL, so make sure
CONFIG_SPL_WDT is not enabled.

Fixes: commit 6874cb72 ("watchdog: Split WDT from SPL_WDT")

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>