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: e4b3cbd ("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>

Author: arndb

drivers/firmware/Kconfig

@@ -215,6 +215,7 @@ config SYSFB_SIMPLEFB
 config TH1520_AON_PROTOCOL
 	tristate "Always-On firmware protocol"
 	depends on ARCH_THEAD || COMPILE_TEST
+	depends on MAILBOX
 	help
 	  Power, clock, and resource management capabilities on the TH1520 SoC are
 	  managed by the E902 core. Firmware running on this core communicates with