Silicon/Socionext/SynQuacer: Casts to avoid void* pointer arithmetic

By default, gcc allows void* pointer arithmetic.
This is a GCC extension.
However, the C reference manual states that void*
pointer "cannot be operands of addition or subtraction
operators". Cf s5.3.1 "Generic Pointers".

This patch adds casts to avoid doing void* pointer arithmetic.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>