board: rpi: add print_archinfo to display arch information

board: rpi: add print_archinfo to display arch information

Current U-boot doesn't display a message about which architecture is
used. So Developer is difficult to know it by intuition.
This patch is displaying to CPU information with CONFIG_SYS_CPU.
- In mainline, it's used to display cpuinfo with CONFIG_DISPLAY_CPUINFO.
  But PRI4 doesn't implement print_cpuinfo(). Instead, display a cpuinfo
  in get_board_rev about RPI4 boards.

In RPI4 case,

U-Boot 2020.10-drity (Jan 15 2021 - 13:24:55 +0900)

DARM:  3.9 GiB
RPI 4 Model B (0xc03111)
CPU: armv7

Change-Id: I485194baa965065f44bac966d014ccfb31b9da60
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>