pmdomain: thead: fix TH1520_AON_PROTOCOL dependency

pmdomain: thead: fix TH1520_AON_PROTOCOL dependency

Kconfig treats the dependency as optional, but the header file only provides
normal declarations and no empty API stubs:

ld: fs/btrfs/extent_io.o: in function `writepage_delalloc':
extent_io.c:(.text+0x2b42): undefined reference to `__udivdi3'
ld: drivers/pmdomain/thead/th1520-pm-domains.o: in function `th1520_pd_power_off':
th1520-pm-domains.c:(.text+0x57): undefined reference to `th1520_aon_power_update'
ld: drivers/pmdomain/thead/th1520-pm-domains.o: in function `th1520_pd_power_on':
th1520-pm-domains.c:(.text+0x8a): undefined reference to `th1520_aon_power_update'
ld: drivers/pmdomain/thead/th1520-pm-domains.o: in function `th1520_pd_probe':
th1520-pm-domains.c:(.text+0xb8): undefined reference to `th1520_aon_init'
ld: th1520-pm-domains.c:(.text+0x1c6): undefined reference to `th1520_aon_power_update'

Since the firmware code can easily be enabled for compile testing, there
is no need to add stubs either, so just make it a hard dependency.

Fixes: dc9a897dbb03 ("pmdomain: thead: Add power-domain driver for TH1520")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
Link: https://lore.kernel.org/r/20250314154834.4053416-1-arnd@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>