clk: sifive: only build sifive-prci.o for CONFIG_CLK_SIFIVE_PRCI

If we're building non FU540/FU740 SoC drivers, then the sifive-prci.o
is not needed. Only build this when CONFIG_CLK_SIFIVE_PRCI is selected.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>