Add PINs init function based on the selected UART port.

In general, user should configure all necessary PINs based on
the specific imxrt board in:
    bsp/imxrt/xxxx/board/MCUX_Config/pin_mux.c
There is a convenient GUI tool named MCUXpresso Config Tools,
it's free and can be obtained from NXP offical website. Open
the file:
    bsp/imxrt/imxrt1052-nxp-evk/board/MCUX_Config/MCUX_Config.mex
with MCUXpresso Config Tools, set the necessary PINs (or clocks)
and generate the new code of pin_mux.c (or clock_config.c), the
project can get right configurations based on specific imxrt
board.

But, there is no relation between RT-Thread Kconfig and MCUXpresso
Config Tools. User selects one driver in RT-Thread menuconfig, then
the ping_mux.c should be udpated by MCUXpresso Config Tools to set
the right PIN configurations for the selected driver. It's clear
but a little complex.

We add the PIN init functions for some drivers in the board.c file,
1. It's convenient to usr a driver in RT-Thread. Just select the
   driver, then the code will include all necessary parts, and the
   driver can work well.
2. User can also configure the PINs with MCUXpresso Config Tools.
3. The drivers in bsp/imxrt/libraries/drivers are common for all
   imxrt boards. They may have different PIN configurations based
   on different hardware design. So we put the PIN init functions
   in each board.c file following each different imxrt board.
Signed-off-by: NGavin Liu <gavin-liugang@outlook.com>