stlink: increase stlink v2 max speed to 2.25 Mbps

stlink: increase stlink v2 max speed to 2.25 Mbps

stlink v2 on Nucleo-64 board (e.g. NUCLEO-L476RG) has target SWO signal
connected to STM32F103CB'S PA10, which is UART1_RX. UART1 within this
MCU in theory can be configured to 4.5 Mbps baudrate, which means this
is the upper limit supported by HW. As a confirmation BMP (Black Magic
Probe) project also states in documentation that UART1 can be used with
up to 4.5 Mbps baudrate.

Tests have shown that configuring 4.5 Mbps baudrate on stlink v2
available on NUCLEO-L476RG board results in receiving corrupted data.
Using 2.25 Mbps however allows to successfully receive all data from
SWO. This makes sense in terms of STM32F103CB capabilities, since 2.25
Mbps is the next supported baudrate due to division by 2.

Increase supported stlink v2 SWO speed from 2 to 2.25 Mbps.

Tested with NUCLEO-L476RG:

  $ stm32l4x.tpiu configure -protocol uart \
    -traceclk 80000000 -pin-freq 2250000 \
    -output /dev/stdout
  $ stm32l4x.tpiu enable

2.25 Mbps speed confirmed with logic analyzer.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>
Change-Id: Icbec04585664aba8b217e8f9a75458e577f7617f
Reviewed-on: https://review.openocd.org/c/openocd/+/7848
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>