target-ppc: create a helper function to allow more flexible RAM allocation for PPC 4xx

The 4xx SDRAM controller supports a small number of banks, and each bank must
be one of a small set of sizes. The number of banks and the supported sizes
varies by SoC.

This function uses the user-specified RAM size to fill in the "ram_bases" and
"ram_sizes" arrays required by ppc4xx_sdram_init().

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6063 c046a42c-6fe2-441c-8c8c-71466251a162