spl: split spl_board_fixups to arch/board specific

spl: split spl_board_fixups to arch/board specific

The current spl_board_fixups API allows for modification of spl_image
before the SPL jumps to it. This can be used to modify the DT for the
next boot stage, however the current API only allows either the machine
arch or the board to use it.

This limits the utility of the API as there might be certain fixups that
should be applied to all boards sharing the same machine architecture
with others being board specific.

For TI's K3 specifically, this prevents us from performing architecture
level fixups since a lot of TI boards are already making use of the
spl_board_fixups API.

Therefore this patch splits the API into two to allow both board and the
architecture specific fixups. The order is kept as arch then board to
give board specific fixups the precedence.

Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Anshul Dalal <anshuld@ti.com>
Tested-by: Wadim Egorov <w.egorov@phytec.de>