ARM: zynq: Remove unused board_early_init_f()

board_early_init_f added by commit e6cc3b25
("arm: zynq: Wire watchdog internals") is no longer needed that's why
remove it also with Kconfig enabling.

Fixes: ccd063e9 ("watchdog: Move watchdog_dev to data section (BSS may not be cleared)")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>