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>

arch/Kconfig

@@ -52,7 +52,8 @@ config SYS_CACHELINE_SIZE
 	default 64 if SYS_CACHE_SHIFT_6
 	default 32 if SYS_CACHE_SHIFT_5
 	default 16 if SYS_CACHE_SHIFT_4
-	# Fall-back for MIPS
+	# Fall-back for MIPS and RISC-V
+	default 64 if RISCV
 	default 32 if MIPS
 
 config LINKER_LIST_ALIGN