ramips: mt7621: disable highmem support and remove highmem offset patch

On MT7621 it was observed, that enabling highmem support causes a significant
performance drop, as documented in: #13151
By adjusting the highmem start offset, we avoid leaving any RAM unaddressable,
even on devices with 512 MB

Fixes: #13151
Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/ramips/mt7621/config-5.15

@@ -7,7 +7,6 @@ CONFIG_ARCH_SUSPEND_POSSIBLE=y
 CONFIG_AT803X_PHY=y
 CONFIG_BLK_MQ_PCI=y
 CONFIG_BOARD_SCACHE=y
-CONFIG_BOUNCE=y
 CONFIG_CEVT_R4K=y
 CONFIG_CLKSRC_MIPS_GIC=y
 CONFIG_CLK_MT7621=y
@@ -91,7 +90,6 @@ CONFIG_HARDWARE_WATCHPOINTS=y
 CONFIG_HAS_DMA=y
 CONFIG_HAS_IOMEM=y
 CONFIG_HAS_IOPORT_MAP=y
-CONFIG_HIGHMEM=y
 CONFIG_I2C=y
 CONFIG_I2C_ALGOBIT=y
 CONFIG_I2C_BOARDINFO=y
@@ -106,7 +104,6 @@ CONFIG_IRQ_DOMAIN_HIERARCHY=y
 CONFIG_IRQ_FORCED_THREADING=y
 CONFIG_IRQ_MIPS_CPU=y
 CONFIG_IRQ_WORK=y
-CONFIG_KMAP_LOCAL=y
 CONFIG_LED_TRIGGER_PHY=y
 CONFIG_LIBFDT=y
 CONFIG_LOCK_DEBUGGING_SUPPORT=y

target/linux/ramips/patches-5.15/120-highmem-start-offset.patch

@@ -0,0 +1,19 @@
+From: Felix Fietkau <nbd@nbd.name>
+Date: Mon Jul 24 13:29:13 CEST 2023
+Subject: [PATCH] mips: ralink: increase highmem start
+
+Increase highmem start address in order to support devices with 512 MB memory
+and MT7621 SoC
+
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+--- a/arch/mips/include/asm/mach-ralink/spaces.h
++++ b/arch/mips/include/asm/mach-ralink/spaces.h
+@@ -5,6 +5,7 @@
+ #define PCI_IOBASE	_AC(0xa0000000, UL)
+ #define PCI_IOSIZE	SZ_16M
+ #define IO_SPACE_LIMIT	(PCI_IOSIZE - 1)
++#define HIGHMEM_START   _AC(0x24000000, UL)
+
+ #include <asm/mach-generic/spaces.h>
+ #endif