fs: Fix SPL build if FS_LOADER is enabled

If FS_LOADER is enabled for the SPL then the build fails with the error:

fs/fs.o:(.data.rel.fstypes+0x128):
undefined reference to `smh_fs_set_blk_dev'
fs/fs.o:(.data.rel.fstypes+0x140):
undefined reference to `smh_fs_size'
fs/fs.o:(.data.rel.fstypes+0x148):
undefined reference to `smh_fs_read'
fs/fs.o:(.data.rel.fstypes+0x150):
undefined reference to `smh_fs_write'

Fix the error by populating the semihosting entry in the fs_types array
only for non-SPL builds.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>