ARM: socfpga: do not add board directory to header search path

The compiler option "-Iboard/$(VENDOR)/$(BOARD)" just exists here
for iocsr_config.c to be able to include iocsr_config.h.

Use "..." instead of <...> to include a header in the same directory.

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