Make the TRNG part of the driver optional, under a boolean config
option.

This makes the driver turris-omnia-mcu available for compilation even if
HW_RANDOM is disabled.

Fixes: ed46f1f7 ("platform: cznic: turris-omnia-mcu: fix Kconfig dependencies")
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240719085756.30598-3-kabel@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>