x86: Refactor table-writing code a little

The implementation of write_tables() is confusing because it uses the
rom_table_start variable as the address pointer as it progresses.

Rename it to rom_addr to make the code clearer. Move the rom_table_end
variable into the block where it is used.

Also update logging to use the ACPI category, now that it is available.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>