swiotbl: add back swiotlb_alloc_boot

This patch makes swiotlb_alloc_boot() available again.

This weak function can be overloaded to modify slightly how the SWIOTLB
and the overflow areas are allocated during boot.

This will be used later to support the Nintendo Wii video game console,
which requires placing the SWIOTLB area above 0x10000000 (MEM2).