acpi: Convert part of acpi_table to use acpi_ctx

The current code uses an address but a pointer would result in fewer
casts. Also it repeats the alignment code in a lot of places so this would
be better done in a helper function.

Update write_acpi_tables() to make use of the new acpi_ctx structure,
adding a few helpers to clean things up.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>