dm: pinctrl: Prevent (re-)configuring pins when already done before relocation

This commit prevents from re-configuring pins if those were configured
before relocation.

Some pins - like UART or DDR must be setup before relocation
(as they have 'u-boot,dm-pre-reloc' property set in DTS). Without this
change, those pins are re-configured after relocation (pre_reloc_only = 0,
so we do not "continue").
Such behavior may be a problem for DDR PAD configuration, as they might
be already leveled/tuned with original setup).

Signed-off-by: Lukasz Majewski <lukma@denx.de>