arm: mvebu: turris_omnia: Increase fdt size in fixup_mtd_partitions

Sometimes fixup_mtd_partitions() prints during booting kernel error
"Failed fixing SPI NOR partitions!" because it does not have enough space
for creating all paritions nodes. So increase fdt size.

Fixes: 92f36c8e ("arm: mvebu: turris_omnia: fixup MTD partitions in Linux' DTB")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Marek Behún <kabel@kernel.org>