Add bounds checking asserts to memmap IO driver

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>