serial: sirf: add a new uart type support

in CSR A7DA SoC, uart6 located at BT module and it need multiple clock
sources, so for "sirf,marco-bt-uart" compatible uarts, drivers take 3
clock sources and enable them.

this patch also replaces clk_get by devm_clk_get function and fix DT
binding document in which we missed to fix when we added marco platform
in commit 909102db

 "serial: sirf: add support for Marco chip".

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>