ufs/ufs.h: Add definition of 'ufshcd_rmwl()'

ufs/ufs.h: Add definition of 'ufshcd_rmwl()'

Add definition of 'ufshcd_rmwl()' helper function
which would be later used by Qualcomm UFS driver
to read-modify-write registers.

Ported from Linux kernel commits:
e785060ea3a1 ("ufs: definitions for phy interface")
cff91daf52d3 ("scsi: ufs: Fix kernel-doc syntax in ufshcd.h")

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Tested-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Tested-by: Julius Lehmann <lehmanju@devpi.de>
Link: https://lore.kernel.org/r/20240930-topic-ufs-enhancements-v3-7-58234f84ab89@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>