MASK_VAL() was added as a way to handle bit_offset and bit_width for
registers located in system memory address space. However, while suited
for reading, it does not work for writing and result in corrupted
registers when writing values with bit_offset > 0. Moreover, when a
register is collocated with another one at the same address but with a
different mask, the current code results in the other registers being
overwritten with 0s. The write procedure for SYSTEM_MEMORY registers
should actually read the value, mask it, update it and write it with the
updated value. Moreover, since registers can be located in the same
word, we must take care of locking the access before doing it. We should
potentially use a global lock since we don't know in if register
addresses aren't shared with another _CPC package but better not
encourage vendors to do so. Assume that registers can use the same word
inside a _CPC package and thus, use a per _CPC package lock.

Fixes: 2f4a4d63 ("ACPI: CPPC: Use access_width over bit_width for system memory accesses")
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Link: https://patch.msgid.link/20240826101648.95654-1-cleger@rivosinc.com
[ rjw: Dropped redundant semicolon ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>