serial: stm32: Properly put Kconfig options in their own submenu

Currently the entries for STM32 UART ports are at the same level than
the top menuconfig UART_STM32. As a result they are mixed with other
configs in the "Serial Drivers" menu.

Use if/endif grammar to put these entries under a dedicated STM32
submenu.

Change-Id: If28945204b801578d29f8cce7c2370ca3c2737a1
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>