clk: samsung: add infrastructure to add separate aliases

The current code adds aliases, if necessary, directly when adding
the clock, limiting the number of possible aliases to one.

Some platforms need more than one alias, like the hsmmc pclocks on
s3c2416 which need a "hsmmc" and "mmc_busclk.0" alias for the s3c-
sdhci driver.

Therefore add the possibility to separately add clock aliases for
previously created clocks.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>