spl: Introduce SoC specific init function

spl: Introduce SoC specific init function

Some architectures use spl_board_init() in their SoC specific
implementation. Board developers should be able to add board specific
implementation via spl_board_init(). Hence, introduce a spl_soc_init()
method which is called right before spl_board_init() for SoC
specific implementation.

Signed-off-by: Lukas Funke <[email protected]>
Reviewed-by: Devarsh Thakkar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Michal Simek <[email protected]>