board_f: cosmetic: change the debug trace to KB in reserve_video

Update the debug trace for the reserved video memory to KB as indicated
in the message with "%luk"; before the patch the computed size
gd->relocaddr - addr is in bytes.

This patch aligns the debug trace in reserve_video() with others
functions, for example on stm32mp157c-dk2:

  - Reserving 3080192k for video at: dfd00000
  + Reserving 3008k for video at: dfd00000
    Reserving 873k for U-Boot at: dfc25000
    Reserving 32776k for malloc() at: ddc23000
    Reserving 72 Bytes for Board Info at: ddc22fb0
    Reserving 280 Bytes for Global Data at: ddc22e90
    Reserving 119072 Bytes for FDT at: ddc05d70
    Reserving 0x278 Bytes for bootstage at: ddc05af0

Fixes: 5630d2fb ("board: Show memory for frame buffers")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>