[ARM] Fix non-page aligned boot time mappings

AT91SAM9260 stopped booting with the recent changes to MM
initialisation - it was asking for a non-aligned virtual address
which caused loops to be non-terminal.  Fix this by rounding
virtual addresses down, but remember to include the offset in
the length, and round the length up to the following page.

This means that asking for a mapping of 4K starting at 2K into
a page maps two pages as one would expect.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>