clk: starfive: jh7110: Separate the PLL driver

Drop the PLL part in SYSCRG driver and separate to be a single
PLL driver of which the compatible is "starfive,jh7110-pll".

Signed-off-by: Xingyu Wu <xingyu.wu@starfivetech.com>
Signed-off-by: Hal Feng <hal.feng@starfivetech.com>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>