doc: release: 3.7: Add note on RaspberryPi Pico related changes

Adds notes on changes about rpi_pico

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

Author: soburi

doc/releases/release-notes-3.7.rst

@@ -382,6 +382,7 @@ Boards & SoC Support
 
     * Added support for :c:func:`sys_arch_reboot()`.
     * :kconfig:option:`CONFIG_RISCV_ISA_EXT_A` is no longer erroneously y-selected.
+  * rp2040: The proprietary UART driver has been discontinued and replaced with PL011.
 
 * Added support for these boards: