ppc4xx: Cleanup Boot/FLASH TLB reassignment for PPC440/460

ppc4xx: Cleanup Boot/FLASH TLB reassignment for PPC440/460

Background Info:
Some PPC440/460 boards have caches enabled in the Boot/FLASH TLB (via
init.S) to speed up the boot process. In relocate_code (start.S) the
cache inhibit attribute for this TLB is set to disable cache. This is
needed for the CFI FLASH driver.

This patch now cleans this code up:
- CONFIG_SYS_TLB_FOR_BOOT_FLASH is defined to 0 (default TLB) if not
  defined in the top of this file. This way, we can remove an ugly
  #ifdef in this code.
- Replace complex "#if defined(CONFIG_440EP) || defined(CONFIG_GR)..."
  statement with "#if defined(CONFIG_440)".
- Remove unnecessary cache invalidate calls resulting in faster bootup.

Signed-off-by: Stefan Roese <sr@denx.de>