tools/socfpgaimage.c: fix build on darwin

tools/socfpgaimage.c: fix build on darwin

socfpgaimage utilizes htole32 and friends, unfortunately these functions are
not available on darwin. Fix it by using the cpu_to_le32 and friends defined
in compiler.h as other parts in mkimage do.

This patch fixes the following error:
---8<---
  HOSTCC  tools/socfpgaimage.o
tools/socfpgaimage.c:77:22: warning: implicit declaration of function 'htole32' is invalid in C99 [-Wimplicit-function-declaration]
        header.validation = htole32(VALIDATION_WORD);
                            ^
tools/socfpgaimage.c:80:22: warning: implicit declaration of function 'htole16' is invalid in C99 [-Wimplicit-function-declaration]
        header.length_u32 = htole16(length_bytes/4);
                            ^
tools/socfpgaimage.c:95:6: warning: implicit declaration of function 'le32toh' is invalid in C99 [-Wimplicit-function-declaration]
        if (le32toh(header.validation) != VALIDATION_WORD)
            ^
tools/socfpgaimage.c:97:6: warning: implicit declaration of function 'le16toh' is invalid in C99 [-Wimplicit-function-declaration]
        if (le16toh(header.checksum) != hdr_checksum(&header))
            ^
4 warnings generated.
...
  HOSTLD  tools/dumpimage
Undefined symbols for architecture x86_64:
  "_htole16", referenced from:
      _socfpgaimage_set_header in socfpgaimage.o
  "_htole32", referenced from:
      _socfpgaimage_set_header in socfpgaimage.o
  "_le16toh", referenced from:
      _verify_buffer in socfpgaimage.o
  "_le32toh", referenced from:
      _verify_buffer in socfpgaimage.o
ld: symbol(s) not found for architecture x86_64
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Pavel Machek <pavel@denx.de>