feat(st-uart): manage oversampling by 8

UART oversampling by 8 allows higher baud rates for UART. This is
required when (UART freq / baudrate) <= 16. In this case the OVER8 bit
needs to be enabled in CR1 register. And the BRR register management is
different:
USARTDIV = (2 * UART freq / baudrate) (with div round nearest)
BRR[15:4] = USARTDIV[15:4]
BRR[3] = 0
BRR[2:0] = USARTDIV[3:0] >> 1

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: Ia3fbeeb73a36a4dc485c7ba428c531e65b6f6c09
1 file changed