OvmfPkg/RiscVVirt: Make SecureBootDefaultKeysInit driver configurable

When Secure Boot is enabled, the SecureBootDefaultKeysInit driver is
not always required. There are alternative methods for enrollment,
such as EnrollDefaultKeys.efi or virt-firmware. Therefore, make the
SecureBootDefaultKeysInit driver an optional build component.

Wrap SecureBootDefaultKeysInit.inf with the SECURE_BOOT_DEFAULT_KEYS
condition, allowing builds to optionally configure the driver.

Signed-off-by: Richard Lyu <richard.lyu@suse.com>