Spreadtrum modem bindings

Required properties:
- compatible: "sprd,modem", "sprd,pcie-modem".
	There will be different modem connection for different project
	"sprd,modem' is used for common soc chip, like sharkl2, sharkl3.
	"sprd,pcie-modem" is used for external modem that is connected
	with AP by pcie, like Roc1 + orca.
- syscon-names: Should contain following entries:
	"shutdowm" for modem shutdown/poweron.
	"deepsleep" for modem deepsleep enable/disable.
	"corereset" for modem core reset hold/release
	"sysreset" for modem system reset hold/release
	"getstatus" for get modem boot status
- syscons: Should contain the base address, offset and bit for each entry in syscon-names.
- 5g-modem-support: Used for support 5g modem.
- poweron-gpio: Used fo extenal modem poweron.
- reset-gpio: Used fo extenal modem reset.
- status: Used to indicate whether enable modem.

Example:

	sprd-modem {
		compatible = "simple-bus";
		ranges;

		lte_pm: modem@0 {
			compatible = "sprd,modem";
			syscons = <&aon_apb_regs 0xff 0x0>,
					<&pmu_apb_regs 0xff 0x100000>,
					<&aon_apb_regs 0x8c 0x1>,
					<&aon_apb_regs 0xff 0x0>,
					<&aon_apb_regs 0xff 0x0>;
			syscon-names = "shutdown", "deepsleep", "corereset",
					"sysreset", "getstatus";
		};

		lte_pubcp: modem@1 {
			compatible = "sprd,modem";
			syscons = <&pmu_apb_regs 0x58 0x2000000>,
				<&pmu_apb_regs 0x544 0x8>,
				<&aon_apb_regs 0x174 0x400>,
				<&pmu_apb_regs 0xb0 0x2>,
				<&aon_apb_regs 0xff 0x0>;
			syscon-names = "shutdown", "deepsleep", "corereset",
					"sysreset", "getstatus";
		};

		nr_dp: modem@1 {
			compatible = "sprd,pcie-modem";
			5g-modem-support;
		};

		nr_pscp: modem@2 {
			compatible = "sprd,pcie-modem";
			5g-modem-support;
		};
	};
