Platform: Remove hardcoded UART baud rate from init

Use the macro DEFAULT_UART_BAUDRATE in stdio_init to set UART baud rate
instead of the hardcoded value 115200.

Change-Id: Id33014f02b39b67f74321e71f082f3a707986e99
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>