dm: core: Show driver name with 'dm tree'

dm: core: Show driver name with 'dm tree'

It is often useful to see which driver was actually selected for each
device. Add a new 'Driver' column to provide this information. Sample
output:

 Class       Probed   Driver     Name
----------------------------------------
 root        [ + ]    root_drive root_driver
 keyboard    [ + ]    i8042_kbd  |-- keyboard
 serial      [ + ]    ns16550_se |-- serial
 rtc         [   ]    rtc_mc1468 |-- rtc
 timer       [ + ]    tsc_timer  |-- tsc-timer
 syscon      [ + ]    ich6_pinct |-- pch_pinctrl
 pci         [ + ]    pci_x86    |-- pci
 northbridge [ + ]    bd82x6x_no |   |-- northbridge@0,0
 video       [ + ]    bd82x6x_vi |   |-- gma@2,0
 vidconsole0 [ + ]    vidconsole |   |   `-- gma@2,0.vidconsole0
...

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>