ANDROID: arm64: fix a mismerge in proc.S

Fix a mismerge in:
"FROMLIST: arm64: mm: avoid x18 in idmap_kpti_install_ng_mappings"

We should write x17 to sctlr_el1, not x18. We observed boot failures
because of this.

Before:
mrs     x17, sctlr_el1
bic     x17, x17, #SCTLR_ELx_M
msr     sctlr_el1, x18

After
mrs     x17, sctlr_el1
bic     x17, x17, #SCTLR_ELx_M
msr     sctlr_el1, x17

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Change-Id: Ib4356ec814beb374b7b57117e029241321f5fc22