MCF532x: make icache_enable use CONFIG_SYS_SDRAM_SIZE

in cpu/mcf532x/start.S, the function icache_enable enables the cache for
a fixed 32MB region starting at the SDRAM start address; this patch
changes the function to cover the region defined by CONFIG_SYS_SDRAM_SIZE

Signed-off-by: Wolfgang Wegner <w.wegner at astro-kom.de>