clk: starfive: Replace SOC_STARFIVE with ARCH_STARFIVE

Using ARCH_FOO symbol is preferred than SOC_FOO.
Set obj-y for starfive/ in Makefile, so the StarFive drivers
can be compiled with COMPILE_TEST=y but ARCH_STARFIVE=n.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Hal Feng <hal.feng@starfivetech.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>