mips: cm: Convert __mips_cm_phys_base() to weak function

Based on the design pattern utilized in the CM GCR base address getter
implementation, the platform-specific code is capable to re-define the
getter and re-use the weakly defined initial version. But since the
pattern hasn't been used for over 10 years and another similar case (CM
L2-sync only base address getter) has just been fixed, let's unify the
interface and convert it to a more traditional single weakly defined
method: mips_cm_phys_base() (see the link below for the discussion around
this).

Link: https://lore.kernel.org/linux-mips/20240215171740.14550-3-fancer.lancer@gmail.com


Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>