arm: socfpga: vining: Fix mtdparts for 2x256 MiB SF variant

arm: socfpga: vining: Fix mtdparts for 2x256 MiB SF variant

The 2x256 MiB SF variant of this system has 192 MiB rootfs MTD partition
containing UBI on SF0, use the correct size in U-Boot environment, else
U-Boot cannot mount UBI and boot on this variant.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tien Fong Chee <tien.fong.chee@intel.com>
Cc: Tom Rini <trini@konsulko.com>