target/ppc: Add TFMR SPR implementation with read and write helpers

target/ppc: Add TFMR SPR implementation with read and write helpers

TFMR is the Time Facility Management Register which is specific to
POWER CPUs, and used for the purpose of timebase management (generally
by firmware, not the OS).

Add helpers for the TFMR register, which will form part of the core
timebase facility model in future but for now behaviour is unchanged.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-ID: <20230625120317.13877-3-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>