header: Add BITS_PER_TYPE()

Upstream commit: a231817c01bda4020632d5c62429e1bdf93f73e5

BITS_PER_TYPE was moved in the upstream kernel in commit 9144d75e22ca
("include/linux/bitops.h: introduce BITS_PER_TYPE") in kernel 4.19 and
is now used by the ath10k driver.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>