Step towards modularizing the Flattened Device Tree code.

Define attributes for each of the specific device bindings: clock,
dai, dma, gpio, i2c, iommu, mbox, mmc_pwrseq, phy, power, power domain,
pwm, regulator, reset controller, spi, system controller, pin
controller. Include these support files only if either a provider
or consumer with one of these attributes is present in the kernel
config.

Add the necessary attributes to the device / attach declarations for
each provider and consumer.

There are some bindings that are consumed by generic code (iommu, pinctrl,
power, power domain). Provide weak stubs for these routines to handle
situations where there is no provider.

No actual code changed; NFCI.

Add support for Xilinx 7-series ADC.

The temperature and voltage sensors are exposed with sysmon:

# envstat -d zynqxadc0
Current CritMax WarnMax WarnMin CritMin Unit
temperature: 67.426 degC
vccint: 1.010 V
vccaux: 1.801 V
vp/vn: 0.011 V
vrefp: 1.250 V
vrefn: 0.004 V
vccbram: 1.009 V
vccpint: 1.009 V
vccpaux: 1.799 V
vcco_ddr: 1.500 V

Add driver for Zynq GPIO controller.

Use generic Arasan SDHCI driver for Zynq-7000.

Add basic Zynq-7000 PS clock subsystem driver.

PR# kern/57068

branches: 1.1.10;
Initial commit of FDTised Xilinx Zynq-7000 support. Tested with

qemu-system-arm -M xilinx-zynq-a9 -dtb zynq-zed.dtb -kernel netbsd.ub \
-serial /dev/null -serial stdio

Based on a set of diffs/files from rjs@

sys/arch/arm/zynq is kept in place to allow existing kernels (e.g.
ZEDBOARD and PARALLELLA) to continue to build/work during the transition,
but eventually sys/arch/arm/zynq will be deleted.