efi: Make FDT extra space configurable

efi: Make FDT extra space configurable

U-Boot currently reserves only 0x3000 bytes when copying the FDT
in copy_fdt(), which may not be sufficient if additional nodes
(such as FMAN firmware) are added later.

This patch uses the exisitng SYS_FDT_PAD to reserve space for FDT fixup
instead of hardcoded value.

This change prevents potential corruption when resizing FDT after
EFI boot, especially when firmware like FMAN requires additional
space.

Signed-off-by: Gabriel Nesteruk <gnesteruk@sii.pl>
Signed-off-by: Pawel Kochanowski <pkochanowski@sii.pl>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>