ARM: 8876/1: fix O= building with CONFIG_FPE_FASTFPE

To use Fastfpe, a user is supposed to enable CONFIG_FPE_FASTFPE
and put downstream source files into arch/arm/fastfpe/.

It is not working for O= build because $(wildcard arch/arm/fastfpe)
checks if it exists in $(objtree), not in $(srctree).

Add the $(srctree)/ prefix to fix it.

While I was here, I slightly refactored the code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>