mtd: phram: Allow multiple phram devices on cmd line

Allow the phram module the ability to create multiple phram mtd
devices via the kernel command line.

Currently the phram module only allows a single mtd device to be
created via the kernel command line. This is due to the phram
module having to store the values until it is initialised
later. This storage is done using a single char[] meaning when
the module_param_call is made the previous value is overidden.

This change modifies the phram system to use a char[][] allowing
multiple devices to be created.

The array size if controlled using the new config option
CONFIG_MTD_PHRAM_MAX_CMDLINE_ARGS that allows the maximum
number of devices to be specified. Currently this option
defaults to a value of 1 leaving the behaviour unchanged.

Signed-off-by: Vishnu Satheesh <vishnu.satheesh@arm.com>