arm64: Fix text patching logic

This patch makes the text patching logic aware of
flag CONFIG_KERNEL_TEXT_RDONLY. The upstream commit
'commit ef59ba11 ("UPSTREAM: arm64: Fix text
patching logic when using fixmap")' does not take
into account KERNEL_TEXT_RDONLY flag while patching
the text section.

CRs-Fixed: 1028648
Change-Id: I19172c60f735ccfda960b6a804bd9b57bb687843
Signed-off-by: Imran Khan <kimran@codeaurora.org>