powerpc: mpc8xx: drop CONFIG_SYS_RESET_ADDRESS

There are no boards that define CONFIG_SYS_RESET_ADDRESS, so drop the
associated mpc8xx code that checks for it.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>