mxc_epdc_pl_hardware: require at least 1 PSU + fix printks

* return -EINVAL from mxc_epdc_pl_hardware if conf->psu_n is 0 or
  greater than 2 as this is an invalid number of HV power
  supplies
* tweak printk messages for consistency and print out the number
  of HV PSUs as well as which options are enabled

Signed-off-by: Guillaume Tucker <guillaume.tucker@plasticlogic.com>
mxc_epdc_fb: fix default VCOM and PSUs in PL hardware init

* if no VCOM voltage is supplied to the module parameters,
  default to 1 PSU in PL hardware configuration
* leave default PL hardware VCOM voltages when none provided as
  module parameters
* tweak printk messages for consistency

Signed-off-by: Guillaume Tucker <guillaume.tucker@plasticlogic.com>