ARM: cygnus: fix link failures when CONFIG_COMMON_CLK_IPROC is disabled

When CONFIG_CYGNUS is set but CONFIG_COMMON_CLK_IPROC is disabled, the
following link failures are caused:

drivers/built-in.o: In function `cygnus_armpll_init':
:(.init.text+0x1d290): undefined reference to `iproc_armpll_setup'
drivers/built-in.o: In function `cygnus_genpll_clk_init':
:(.init.text+0x1d2c4): undefined reference to `iproc_pll_clk_setup'
drivers/built-in.o: In function `cygnus_lcpll0_clk_init':
:(.init.text+0x1d304): undefined reference to `iproc_pll_clk_setup'
drivers/built-in.o: In function `cygnus_mipipll_clk_init':
:(.init.text+0x1d344): undefined reference to `iproc_pll_clk_setup'
drivers/built-in.o: In function `cygnus_asiu_init':
:(.init.text+0x1d370): undefined reference to `iproc_asiu_setup'

It is fixed it by always selecting COMMON_CLK_IPROC from
ARCH_BCM_IPROC, and making COMMON_CLK_IPROC a silent option (thus
preventing it from being erroneously disabled by a user).

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>