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>
diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 77ee897..992aca1 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -120,7 +120,7 @@
 
 -  **#define : CACHE_WRITEBACK_GRANULE**
 
-   Defines the size in bits of the largest cache line across all the cache
+   Defines the size in bytes of the largest cache line across all the cache
    levels in the platform.
 
 -  **#define : FIRMWARE_WELCOME_STR**