arm: socfpga: Make the pinmux table const u8

Now that we're actually converting the QTS-generated header files,
we can even adjust their data types. A good candidate for this is
the pinmux table, where each entry can have value in the range of
0..3, but each element is declared as unsigned long. By changing
the type to u8, we can save over 600 Bytes from the SPL, so do it.
This patch also constifies the array.

Signed-off-by: Marek Vasut <marex@denx.de>