spl: remove dependency to "asm/utils.h"

Header file "asm/utils.h" is not required to compile "spl_ymodem.c".
So, removing this dependency allows other architectures to use this
booting device as "asm/utils.h" is only present in "arm" architecture.

Signed-off-by: Paulo Zaneti <paulo.zaneti@datacom.ind.br>