docs(porting-guide): correct typo of "bits" to "bytes"

docs(porting-guide): correct typo of "bits" to "bytes"

The CACHE_WRITEBACK_GRANULE is documented to be in bits, but
specifying the value in bits broke a build. Further investigation
suggests that the value should in fact be in bytes. See
https://github.com/ARM-software/arm-trusted-firmware/blob/master/include/arch/aarch64/
smccc_helpers.h#L101
and
https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Common-Type-Attributes.html

Change-Id: I9a2b2fbe18d5a58a8f9aeb2726a0623f3484c88e
Signed-off-by: Max Yu <maxlyu@google.com>