This change is required to sucessfully use PA13/PA14 for UART. Otherwise
they function as SWDIO/SWCLK.
Relevant excerpt from the reference manual (translated):
The initial state of PA13, PA14, PA15, PB3, and PB4 ports is that the
JTAG/SWD function is valid after reset. When configuring FSEL[5:0] to
select the function, you need to write 0 to the corresponding bit of the
register PSPCR to invalidate the JTAG/SWD function.
Signed-off-by: Boris-Chengbiao Zhou <bobo1239@web.de>
