riscv: vdso: Untangle Kconfig logic

On riscv32 the generic vDSO infrastructure is used but without its
time-related functionality. The Kconfig logic to implement this
treats HAVE_GENERIC_VDSO as a synonym for GENERIC_GETTIMEOFDAY.
This works today due to some underlying issues in how the generic vDSO
library works. Some future cleanups will break this logic.

Restructure the Kconfig logic, so HAVE_GENERIC_VDSO refers to the generic
library in general and GENERIC_GETTIMEOFDAY refers to its time-related
functionality.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250826-vdso-cleanups-v1-6-d9b65750e49f@linutronix.de