pinctrl: bcm2712: Fix for the first valid GPIO

A non-zero mux bit number is used to detect a valid entry in the
pin_regs tables, but GPIO 0 (GPIO 1 on D0) is a valid GPIO with a mux
bit number of zero, so add a high-bit on all valid entries to
distinguish this from an uninitialised row in the table.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>