arm64/sysreg: Remove duplicate definitions from asm/sysreg.h

With the new-fangled generation of asm/sysreg-defs.h, some definitions
have ended up being duplicated between the two files.

Remove these duplicate definitions, and consolidate the naming for
GMID_EL1_BS_WIDTH.

Signed-off-by: Will Deacon <will@kernel.org>