riscv: simplify Kconfig magic for 32-bit vs 64-bit kernels

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#	Subject and files	Size	Replies
1/3	riscv: simplify Kconfig magic for 32-bit vs 64-bit kernels`arch/riscv/Kconfig`	+6 ~~-25~~	0 replies
2/3	riscv: only enable ZONE_DMA32 for 64-bit`arch/riscv/Kconfig`	+1 -1	0 replies
3/3	riscv: add swiotlb support`arch/riscv/Kconfig` `arch/riscv/include/asm/dma-mapping.h` `arch/riscv/kernel/setup.c`	+18 -0	0 replies

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