armV7R: K3: am654: Allow using SPL BSS pre-relocation

armV7R: K3: am654: Allow using SPL BSS pre-relocation

In order to be able to use more advanced driver functionality which often
relies on having BSS initialized during early boot prior to relocation
several things need to be in place:

1) Memory needs to be available for BSS to use. For this, we locate BSS
   at the top of the MCU SRAM area, with the stack starting right below
   it,
2) We need to move the initialization of BSS prior to entering
   board_init_f(). We will do this with a separate commit by turning on
   the respective CONFIG option.

In this commit we also clean up the assignment of the initial SP address
as part of the refactoring, taking into account the pre-decrement post-
increment nature in which the SP is used on ARM.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>