KEYPAD: Document for the Phytium keypad port.

phytium inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8AOG1


CVE: NA

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This is doc for the Phytium Keypad port.

Signed-off-by: Song Wenting <songwenting@phytium.com>
Signed-off-by: Chen Baozi <chenbaozi@phytium.com.cn>
Signed-off-by: lishuo <lishuo@phytium.com.cn>