soc: loongson: loongson_pm2: Add dependency for INPUT

Since commit 67694c076bd7 ("soc: loongson2_pm: add power management
support"), the Loongson-2K PM driver was added, but it didn't update the
Kconfig entry for the INPUT dependency, leading to build errors, so
update the Kconfig entry to depend on INPUT.

/opt/crosstool/gcc-13.2.0-nolibc/loongarch64-linux/bin/loongarch64-linux-ld:
drivers/soc/loongson/loongson2_pm.o: in function `loongson2_power_button_init':
/work/lnx/next/linux-next-20230825/LOONG64/../drivers/soc/loongson/loongson2_pm.c:101:(.text+0x350): undefined reference to `input_allocate_device'
/opt/crosstool/gcc-13.2.0-nolibc/loongarch64-linux/bin/loongarch64-linux-ld:
/work/lnx/next/linux-next-20230825/LOONG64/../drivers/soc/loongson/loongson2_pm.c:109:(.text+0x3dc): undefined reference to `input_set_capability'
/opt/crosstool/gcc-13.2.0-nolibc/loongarch64-linux/bin/loongarch64-linux-ld:
/work/lnx/next/linux-next-20230825/LOONG64/../drivers/soc/loongson/loongson2_pm.c:111:(.text+0x3e4): undefined reference to `input_register_device'
/opt/crosstool/gcc-13.2.0-nolibc/loongarch64-linux/bin/loongarch64-linux-ld:
/work/lnx/next/linux-next-20230825/LOONG64/../drivers/soc/loongson/loongson2_pm.c:125:(.text+0x3fc): undefined reference to `input_free_device'
/opt/crosstool/gcc-13.2.0-nolibc/loongarch64-linux/bin/loongarch64-linux-ld: drivers/soc/loongson/loongson2_pm.o: in function `input_report_key':
/work/lnx/next/linux-next-20230825/LOONG64/../include/linux/input.h:425:(.text+0x58c): undefined reference to `input_event'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>