fix(brcm): fix the build failure with mbedTLS config

fix(brcm): fix the build failure with mbedTLS config

Patch [1] introduces a mechanism to provide the platform
specified mbedTLS config file, but that result in build failure
for Broadcom platform.
This build failure is due to the absence of the mbedTLS configuration
file i.e. brcm_mbedtls_config.h in the TF-A source code repository.
"fatal error: brcm_mbedtls_config.h: No such file or directory"

This problem was resolved by removing the 'brcm_mbedtls_config.h' entry
from the broadcom platform makefile, allowing this platform to use
the default mbedtls_config.h file.

[1]: https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/13726

Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Change-Id: I7cc2efc049aefd3ebce1ae513df9b265fe31ded6