powerpc: mpc52xx: Clear GD in the main U-Boot stage for SPL usage

When an MPC5200 based board is used with SPL support, the main
U-Boot needs to clear the GD (global data) struct again.

Otherwise the generic board init code in board_init_f (when
CONFIG_SYS_GENERIC_BOARD is defined) will not initialize all
GD variables correctly. Resulting in a hangup on the a4m2k
board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>