docs: Add device tree bindings for SBI PMU extension

SBI PMU extension requires a firmware to be aware of the event to
counter/mhpmevent mappings supported by the hardware. One approach
is to encode that information in the device tree.

Define a device tree binding that allows a hardware to describe
all the PMU mappings required in concise format.

Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>