irqchip/mbigen: rename register marcros

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8NULW


CVE: N/A

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A MBIGEN can also be used for generating SPIs, so let's
rename register macros to make them more resonable.

The first 64-pins of MBIGEN is used by SPIs, so rename
RESERVED_IRQ_PER_MBIGEN_CHIP to SPI_NUM_PER_MBIGEN_CHIP
and change the comment for this marcro.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Yuntao Liu <liuyuntao12@huawei.com>