m68k/atari - stram: alloc ST-RAM pool even if kernel not in ST-RAM

m68k/atari - stram: alloc ST-RAM pool even if kernel not in ST-RAM

With the kernel loaded to FastRAM (TT-RAM), none of the ST-RAM
address range is mapped by init_mem, and ST-RAM is not accessible
through the normal allocation pathways as a result.

Implement ST-RAM pool allocation to be based on physical addresses
always (it already was when the kernel was loaded in ST-RAM).
Return kernel virtual addresses as per normal.

The current test for the kernel residing in ST-RAM always returns
true. Use the bootinfo memory chunk order instead - with the kernel
in FastRAM, ST-RAM (phys. 0x0) is not the first chunk.

In case the kernel is running from FastRAM, delay mapping of ST-RAM
pool until after mem_init.

Provide helper functions for those users of ST-RAM that need
to be aware of the backing physical addresses.

Kudos to Geert for his hints on getting this started.

Signed-off-by: Michael Schmitz <schmitz@debian.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>