spl: Drop bd_info in the data section

spl: Drop bd_info in the data section

This uses up space in the SPL binary but it always starts as zero. Also
some boards cannot support data in TPL (e.g. Intel Apollo Lake).

Use malloc() to allocate this structure instead, by moving the init a
little later, after malloc() is inited. Make this function optional since
it pulls in malloc().

This reduces the TPL binary size on coral by about 64 bytes

Signed-off-by: Simon Glass <sjg@chromium.org>