avb: Fix build when CONFIG_OPTEE_TA_AVB is disabled

avb: Fix build when CONFIG_OPTEE_TA_AVB is disabled

When having only these AVB related configs enabled:

    CONFIG_AVB_VERIFY=y
    CONFIG_CMD_AVB=y
    CONFIG_LIBAVB=y

build fails with next errors:

    common/avb_verify.c: In function 'read_persistent_value':
    common/avb_verify.c:867:6: warning: implicit declaration of function
                                        'get_open_session'
    common/avb_verify.c:870:45: error: 'struct AvbOpsData' has no member
                                        named 'tee'
    common/avb_verify.c:894:7: warning: implicit declaration of function
                                        'invoke_func'
    common/avb_verify.c: In function 'write_persistent_value':
    common/avb_verify.c:931:45: error: 'struct AvbOpsData' has no member
                                       named 'tee'

Guard read_persistent_value() and write_persistent_value() functions
by checking if CONFIG_OPTEE_TA_AVB is enabled (as those are only used in
that case) to fix the build with mentioned configuration.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>