PPC: fix hreset_vector for 60x, 7x0, 7x5, G2, MPC8xx, MPC5xx, 7400 and 7450

According to the different user's manuals, the vector offset for system
reset (both /HRESET and /SRESET) is 0x00100.

This patch may break support of some executables, as the power-on start
address may change. For a specific board, if the power-on start address
is different than HRESET vector (i.e. 0x00000100 or 0xfff00100), this
should be fixed in board's initialization code.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Alexander Graf <agraf@suse.de>