juno: Add PSU channels support for juno XRP7724

This patch adds support for the PSU driver of the
XRP7724 component of the juno board.

It provides an implementation for the PSU driver API and
uses the I2C HAL API to make PSU request to the XRP7724
component.

Change-Id: I2c0b910307de92e49f23336ca16606c1cf12fbcb
Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>
Signed-off-by: Jim Quigley <jim.quigley@arm.com>