drm/amdgpu/powerplay: add some documentation about memory clock

drm/amdgpu/powerplay: add some documentation about memory clock

We expose the actual memory controller clock rate in Linux,
not the effective memory clock of the DRAMs.  To translate
it, it follows the following formula:

Clock conversion (Mhz):
HBM: effective_memory_clock = memory_controller_clock * 1
G5:  effective_memory_clock = memory_controller_clock * 1
G6:  effective_memory_clock = memory_controller_clock * 2

DRAM data rate (MT/s):
HBM: effective_memory_clock * 2 = data_rate
G5:  effective_memory_clock * 4 = data_rate
G6:  effective_memory_clock * 8 = data_rate

Bandwidth (MB/s):
data_rate * vram_bit_width / 8 = memory_bandwidth

Some examples:
G5 on RX460:
memory_controller_clock = 1750 Mhz
effective_memory_clock = 1750 Mhz * 1 = 1750 Mhz
data rate = 1750 * 4 = 7000 MT/s
memory_bandwidth = 7000 * 128 bits / 8 = 112000 MB/s

G6 on RX5600:
memory_controller_clock = 900 Mhz
effective_memory_clock = 900 Mhz * 2 = 1800 Mhz
data rate = 1800 * 8 = 14400 MT/s
memory_bandwidth = 14400 * 192 bits / 8 = 345600 MB/s

Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>