arm: stm32mp: support several key in command stm32key

Update the command stm32key to support several keys selected by
key name and managed by the new sub-command:

stm32key list
stm32key select [<key>]
stm32key read -a

This patch doesn't change the STM32MP15 behavior, only PKH is
supported, but it is a preliminary patch for STM32MP13 support.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>