soc_core: csr_alignment assertions

soc_core: csr_alignment assertions

Enforce the condition that csr_alignment be either 32 or 64 when
requested explicitly when initializing SoCCore().

Additionally, if a CPU is specified, enforce that csr_alignment be
equal to the native CPU word size (currently either 32 or 64), and
warn the caller if an alignment value *higher* than the CPU native
word size was explicitly requested.

In conclusion, if a CPU is specified, then csr_alignment should be
assumed to equal 8*sizeof(unsigned long).

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>