uniphier: set buffer offset and length for io_block dynamically

Currently, the .buffer field in io_block_dev_spec is statically set,
which is not handy for PIE.

Towards the goal of making this really position-independent, set the
buffer length and length in the uniphier_io_block_setup() function.

Change-Id: I22b20d7b58d6ffd38f64f967a2820fca4bd7dade
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>