clk: aspeed: Mark bclk (PCIe) and dclk (VGA) as critical

This is used by the host to talk to the BMC's PCIe slave device. The BMC
is not involved, but the clock needs to be enabled so the host can use
the device.

Fixes: 15ed8ce5 ("clk: aspeed: Register gated clocks")
Cc: stable@vger.kernel.org # 4.15
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Tested-by: Lei YU <mine260309@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>