fdt_support: call mtdparts_init() after finding MTD node to fix up

Platform code can call fdt_fixup_mtdparts() in order to hand U-Boot's
MTD partitions over to the Linux device tree.

Currently, fdt_fixup_mtdparts() calls mtdparts_init() in its entry.
If no target MTD device is found, an error message like follows is
displayed:

    Device nand0 not found!

This occurs when the same code (e.g. arch/arm/mach-uniphier/fdt-fixup.c)
is shared among several boards, but not all of them support an MTD device.

Parse the DT first, then call mtdparts_init() only when the target MTD
node is found.

Yet, you still need to call mtdparts_init() before device_find().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>