powerpc/mpc85xx: modify the functionality clear_bss and aligning the end address...

powerpc/mpc85xx: modify the functionality clear_bss and aligning the end address of the BSS

There will clear the BSS in the function clear_bss(), the reset address of
the BSS started from the __bss_start, and increased by four-byte increments,
finally stoped depending on the address is equal to the _bss_end. If the end
address __bss_end is not alignment to 4byte, it will be an infinite loop.

1. The reset action stoped depending on the reset address is greater
than or equal the end address of the BSS.
2. The end address of the BSS should be 4byte aligned. Because the reset unit
is 4 Bytes.

This patch is on top of the patch "powerpc/mpc85xx: support application
without resetvec segment in the linker script".

Signed-off-by: Ying Zhang <b40530@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>