ARM: uniphier: set DTB file name to fdt_file environment

ARM: uniphier: set DTB file name to fdt_file environment

When we want to boot Linux with a DTB file downloaded from a TFTP
server or somewhere, we need to know the file name to be downloaded.

Assume the U-Boot configuration is shared among some similar boards.
If they are similar enough, the difference only appears in device
trees.  The build procedure would be like this:

 - Board A:  make foo_common_defconfig && make DEVICE_TREE=foo_board_a
 - Board B:  make foo_common_defconfig && make DEVICE_TREE=foo_board_b
 - Board C:  make foo_common_defconfig && make DEVICE_TREE=foo_board_c

In this case, the U-Boot image contains nothing about the DTB file name
it is running with.  (CONFIG_DEFAULT_DEVICE_TREE is not helpful for this
purpose because it is painful to change it from "make menuconfig" for
each board.)

This commit allows to lookup the DTB file name based on the compatible
string and set it to "fdt_file" environment.  Then "tftpboot $fdt_file"
will download the file we want.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>