ARM: 6192/1: VExpress: Align the machine_desc.phys_io to 1MB section

When not aligned, random bits could be written in the initial page table
by the __create_page_tables() function.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>