firmware: thead: add CONFIG_MAILBOX dependency

firmware: thead: add CONFIG_MAILBOX dependency

Without this, the driver fails to build:

ld: drivers/firmware/thead,th1520-aon.o: in function `th1520_aon_call_rpc':
thead,th1520-aon.c:(.text+0x28): undefined reference to `mbox_send_message'
ld: drivers/firmware/thead,th1520-aon.o: in function `th1520_aon_deinit':
thead,th1520-aon.c:(.text+0x17e): undefined reference to `mbox_free_channel'
ld: drivers/firmware/thead,th1520-aon.o: in function `th1520_aon_init':
thead,th1520-aon.c:(.text+0x1d9): undefined reference to `mbox_request_channel_byname'

Fixes: e4b3cbd840e5 ("firmware: thead: Add AON firmware protocol driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
Link: https://lore.kernel.org/r/20250314154816.4045334-1-arnd@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>