Introduce variables to deal with bit layout differences in the UFSTAT

Introduce variables to deal with bit layout differences in the UFSTAT
register.  Use 32-bit reads and writes to access the URXH and UTXH
registers.  They're documented as 32-bit registers in the Exynos 4 and
Exynos 5 User Manuals and accessing URXH with an 8-bit read triggers a
fault on Apple's M1 SoC.

ok patrick@