[POWERPC] PS3: Bootwrapper improvements

Improve the debugging support of the PS3 bootwraper code:

 o Increase the size of the PS3 bootwrapper overlay from 256 to 512 bytes to
   allow for more debugging code in the overlay.
 o Use the dot symbol to set the size of __system_reset_overlay.  The
   assembler will then emit an error if the overlay code is too big.
 o Remove some unused instructions.
 o Update the text describing the PS3 bootwrapper overlay.
 o Add a check for null pointer writes.
 o Change hcall return value from s64.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>