soc: kunpeng_hccs: add MAILBOX dependency

When the mailbox subsystem is disabled, the driver fails to link, so add
an explicit dependency. Some other drivers use 'select' here, but the
dependency seems more logical and more common.

Fixes: 886bdf9c ("soc: hisilicon: Support HCCS driver on Kunpeng SoC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>