stm32f1: rcc: timer 15,16,27 rcc bits missing for old style

The bits definitions for direct manipulation were missing, and should be
present for completeness.  However, this only affects the legacy (error prone)
API, replaced some time ago.

old and error prone: (stop using code like this)
    rcc_peripheral_enable_clock(&RCC_APB2ENR, RCC_APB2ENR_TIM16EN);
    rcc_peripheral_reset(&RCC_APB2RSTR, RCC_APB2RSTR_TIM16RST);

new:
    rcc_periph_clock_enable(RCC_TIM16);
    rcc_periph_reset_{pulse,hold,release}(RCC_TIM16);

Fixes github issue #361