The memmap IO driver doesn't perform bounds check when reading, writing,
or seeking. The onus to vet parameters is on the caller, and this patch
asserts that:

  - non-negative size is specified for for backing memory;

  - valid parameters are passed into the driver for read, write and seek
    operations.

Change-Id: I6518c4065817e640e9e7e39a8a4577655f2680f7
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>