rockchip: merge misc.c into board.c

The functions aren't used anywhere else than in board.c, therefore,
let's not expose them anymore at all.

This merges misc.c and board.c together and removes the functions from
the misc.h header file.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>