spi: loongson: Add LS7A spi driver support

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BWFP



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Change-Id: Ib4552613f7c7706a0773839ddec7384d4a1d40a0
Signed-off-by: Juxin Gao <gaojuxin@loongson.cn>
(cherry picked from commit 9f0b0ecf)