Kconfig: Add a default cache line size for RISC-V

Kconfig: Add a default cache line size for RISC-V

The RISC-V ISA profile RVA23U64 requires extension Zic64b (Cache blocks
must be 64 bytes in size, naturally aligned in the address space).

Some RISC-V platforms do not define the d-cache line size through SYS_CACHE_SHIFT_n.
Set a default value of 64 bytes for such cases.

Signed-off-by: Yu-Chien Peter Lin <peter.lin@sifive.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>