Fix compressed ROM support
When PhysRamTable was updated to store addresses in page units instead of byte units (commit df4efb68), the code which allocates the ROM decompression workspace didn't get updated, causing it to break. Add a few extra shifts to the code in order to account for the changes.
Fixes issue reported on forums with (compressed) OMAP3 ROM failing to boot: https://www.riscosopen.org/forum/forums/5/topics/16446